Alien Invasion:- Will it ever happen?, and would we stand a chance?

So, after watching the final episode of the current BBC Three series "Conspiracy Road trips", which centred on the belief that UFO/Alien cover ups exist, one thing stood out to me in particular.
Amongst the "road tippers", which was the usual assortment of overly paranoid conspiracy buffs and new age-esque hippy types who claim some form of alien contact, there was a man who made the astonishing claim that he had been personally attacked by aliens in his own home, an encounter which he said he would do everything in his power to prevent again.
The most worrying aspect of his story, which involved him being supposedly set upon by a group of alien creatures while he was wearing only his underpants (!?!) was that after this supposed attack, he had developed an extremely paranoid attitude towards the whole thing, culminating in him showing on screen the collection of edged weapons he keeps hidden inside his house (which he shares with his wife and two young children), including a vicious looking short sword kept down the side of his sofa, and an axe kept hidden in his bedroom, and stating his intent to use them should any alien enter his property again. He was also later shown to be interested in purchasing various firearms when the group visited as gun shop during the course of their road trip (which, as any resident of the UK will know, firearms are generally illegal to obtain or own throughout the entire British isles).
Mental illness issues and suitability to be allowed access to children aside, It got me thinking about what would actually happen if the EEBUL ALEONZ!!11!! actually did turn up on our doorstep and announced that they were spoiling for a rumble.

So, this thought experiment (of sorts) makes the following assumptions:-

#1 - The aliens come from a planet comparable to ours, within 10 light years of Earth
#2 - The aliens are biologically comparable to us, as in oxygen/nitrogen breathing with similar tolerances to gravity, atmospheric pressure and heat/cold.
#3 - The aliens are more technologically advanced than us. (This is a given being as that with our current level of technology, interstellar travel is not available to humans)  
 
So, as in law, there are three sections that need to be covered, "Means", "Motive" and "Opportunity".

Opportunity

I'll cover this one first, as it is probably the simplest.
As it stands right now, the human race doesn't look much further than its own doorstep. We have various methods of observing the space around us, such as telescopes of both the optical and radio types, as well as radar, lidar and so on, but as routine, we do not maintain a permanent observation of space in its totality.
It could come to pass that a comet or asteroid could appear in one of our blind spots and annihilate us with very little warning, so thus, it could also be conceivable that an alien spacecraft or fleet could sneak up on us unnoticed.
Since the beginning of the 20th century, humans have been beaming various radio and TV signals into space (both intentionally as well as unintentionally), as well as hurling unmanned satellites into the void. Granted, most satellites sent into space these days lack the indentificaton markers and "invitations to visit" that some of the probes sent out in the 60s and 70s had (i.e, the Voyager golden record or the Pioneer plaque, both of which provide reference points by which the finder could use to see the probes point of origin), but as it stands, the human race has been very noisy over the past 150 years or so, and to anyone who has been listening or watching, the idea of coming here may seem attractive, especially if they have the capability, and if they have malevolent intent, the chances are that they could arrive and take us more or less completely by surprise.
"Officially" we do not have any kind of orbital weapons systems, nor do we have any kind of space combat vessels, so should a fleet of alien spacecraft suddenly pop up out of nowhere, our chances of being able to halt them before they get too close are practically nil, a rather depressing thought, almost akin to leaving your backdoor  wide open with all your valuables laid out and neatly catalogued on the kitchen table, along with a note saying "please take all my stuff, KTHXBAI!"

Of course, as there is very little evidence to actually support the existence of any kind of intelligent alien life, not being prepared for an alien attack could be seen as simple common sense, after all, why be paranoid about a risk we are fairly sure doesn't exist?, add to this formula mans arrogance that something like that could never possibly happen, and you can see why there is very little interest in the subject at anything more than a speculative level.
The problem with that kind of thinking though is that mathematically speaking, it is improbable in the extreme for there NOT to be some other kinds of intelligent life out there..hmm.


Motive

So, after we have announced our presence to anyone who happens to be paying attention, and they decide to pay us a visit, what would their intentions be?.
Well, as with many things, it can either go good or bad.
Given that this race had the technological means to get from their planet to ours in some manner, whereas we don't have the power to go see them, this would mean that they would have to be more technologically advanced than us in the field of space travel, but their ability to do a thing would be tempered by their reasons for doing said thing: -

Peaceful intent

Its not outside the realms of possibility that a more technologically and culturally advanced society would see contact with another intelligent species as a source of great excitement and curiosity, and would visit us with the intention of wanting to learn more about us as a species on a purely cultural level. They may also wish to trade or even assist us with our problems by giving us access to their superior knowledge.
The problem with this is that should this happen, it would be akin to times when a "superior" race of people imposes itself on the culture of another, for example, the British, French and Italian empires expansion into Africa caused many problems such as giving people who were at a lesser stage of societal, cultural and technological development access to weapons and knowledge that they were not able to handle in a responsible manner, the repercussions of which can still be seen today.
Should aliens visit Earth and allow us access to their technological and/or medical advancements, it wouldn't be long, barring some sort of massive cultural change, before mankind's inherent jealousies and pettiness would lead to someone somewhere figuring out a way to weaponise the new technologies.
For example, if the aliens showed us how to build practical small sized fusion or anti matter generators or batteries, it wouldn't be long before someone figured out that you could connect these up to serve as a power source for the many directed energy weapons that we have already developed but as yet have not been able to find a practical power source for.
Medical science could be corrupted to produce any one of a number of noxious chemical or biological weapons, which when bolted on to missiles would form very potent means of exterminating another nation in a single strike and not leave any nasty radioactive crap floating around like what you get with good old fashioned nuclear weapons.
Teleporters could be used to dispense with the need for missiles and bombers and transport bombs instantaneously to their targets without any kind of advance warning.

The list goes on, but even if aliens came with nothing but good intentions, the chances are that in some way, humans would fuck it all up, simply because we aren't at the stage yet where we can handle not being the top of the pile, much less put aside our differences and make a big step in our understanding of the universe at large.

Malevolent intent

There are varying degrees of malevolence, depending on what the aliens actual objectives are:-

Resources (mineral) - The Earth is abundant with many metals, chemicals and building materials, most notably oxygen, aluminium, silicon, iron, calcium, sodium, copper, strontium, uranium, germanium and so on, all of which have many uses within a sufficiently advanced culture. In addition to this, being as that 71% of the Earths surface is covered with water (DiHydrogenOxide or H2O), which is essential for all known forms of life on Earth, it is surmisable that an alien species similar to us would also require water for some reason.

Resources (organic) - As of the time of writing, there are nearly seven billion (7,000,000,000 short scale) humans living on Earth, in addition to the billions of individual examples of animal, plant and bacteriological life forms we share the planet with. All of these life forms contain various proteins, amino acids and other "useful" compounds and elements that could be rendered down and used as food, fertilisers or many other chemical uses.
If mass slaughter isn't their thing, 7 billion humans would make a handy dandy workforce if they could be subjugated, even if a 20-50% casualty rate stemming from a forceful invasion thinned out numbers out, that would still leave between between 3.5 and 5 billion pairs of hands left to mine resources for our new overlords, or shipped off world to work somewhere else.

Conquest (benevolent) - Our visitors may see us as an unruly child race who is in dire need of a good stern lesson in humility, and thus come to Earth to stop us from inevitably causing trouble.
Alternatively, the aliens may see the damage we are causing to our own world, and decide to take steps to preserve the rarity of a world which can support life of a similar type to theirs by either annihilating us or at the very least subjugating us "for our own good".

Conquest (malevolent) - A warlike, expansionist or nomadic race may routinely seek out and invade worlds which are able to support lifeforms such as theirs, with a view to either colonising them or using them as a base from which to move on to the next world, in addition to using them as a place to resupply themselves with the essential resources required for conquest.


Needful intent

It may be a strange idea, but its not outside the realms of possibility that an alien race may come here because they have no permanent home of their own, or because we have something they need in order to fix problems on their own world.

Nomads/homeless - For whatever reason, the aliens home world may have been destroyed or otherwise rendered uninhabitable, and a percentage of their civilisation was able to escape into space to begin looking for a new home. Their arrival on an already overcrowded Earth would in all likelihood be met with both welcoming arms and open hostility, as they would represent both confirmation that man is not alone but also would end up being more mouths to feed, which again, could lead to problems integrating them into our society and cultures, possibly ending up with some sort of global war of some description, but certainly inaugurating a new level of prejudice based on species rather than skin colour, religion (or lack thereof), social status and so on.

Needy/beggars - As I said, Earth has abundances of many resources, any of which could be useful to aliens even in small (considering) quantities. For example, water, which is one of the most abundant things on the face of the Earth (all 71% of it, in addition to the unconfirmed amounts known to exist on the Moon and Mars, and the suspected amount of it on Jupiter's moons) could be required due to their planets supply either being contaminated in some way, or because their worlds water supply is limited and over population means they need to find more from somewhere.


Means

So, the aliens have now figured out why and when they will come, but now its time to ask how they would come and what they would do when they get here, there are many options available:-

Travel
As the aliens are more advanced than us, there are any number of ways they could come here, using both "conventional" and/or "theoretical" means...

Generational Ship/Fleet (sublight) - A single large ship, or a fleet of ships, carrying everything a conquering force would need in order to carry out a perceived successful invasion. In essence, this ship/fleet would have to consist of both an invasion force and a colonisation/occupation force, meaning that there would have to be a hell of a lot of aliens to make it worthwhile. for example, if they decided that for every 10 humans there would be the need for 1 alien,  that would mean that somehow they would need to ensure that by the time they got here, they had at least 7,000,000,00 aliens ready to fight, in addition to weapons, vehicles, equipment and mundane things like food, medicines, clothing (if they require any).
If the vessel could not travel at or faster than the speed of light, then this journey would be very long, and very tedious if they did not have access to some kind of method of suspended animation, the food, drink and medicine requirements alone would be a vast logistical nightmare which would more than likely require centuries of planning and preparation in order to successfully pull off. Add to this the fact that you'd need to keep everyone on the fleet/ship entertained and educated while the journey is taking place (in order to both keep them sane and to ensure that they don't stray from their purpose), and the whole "sublight invasion fleet" idea gets a bit too ridiculous for even a desperate race to consider.
Having access to some kind of cryogenic/suspended animation technology would significantly ease the logistical requirements relating to the welfare of the voyagers, as they wouldn't need to be fed, clothed, medicated or entertained during the journey, although there would in all likelihood be a requirement for some to remain awake and die in space in order to keep the ships systems maintained etc, although this function could be carried out by robots or automated repair systems of some kind.

 Generational Ship/Fleet (light speed) - If the planet of origin is 10 light years away, then if the ships can travel at the speed of light, the journey will take roughly 10 years (given the requirement for time needed to accelerate and then decelerate, provided they didn't have the ability to stop/start without accelerating), roughly equivalent to a single generation. This of course does ease the logistics of the effort somewhat, because food for 10 years would have to be provided, as well as all the other required equipment as mentioned before. Access to some method of suspended animation would eliminate much of the need for this, but would still require the transportation of 7,000,000,00 aliens for the assault.

Wormhole/Faster than light/Space folding travel - Being able to cut down travel time from the order of centuries to a matter of months, or even weeks, simplifies the logistics problems which would otherwise have been encountered. Also, as human detectors can only "see" at the speed of light, something travelling faster than light,  or something which simply appears seemingly out of nowhere would provide an additional element of surprise.

Assault

Once within striking distance, the invaders would of course need to attack in some way. As a species, having very little by way of space based offensive capability, we in all likelihood would not be able to prevent or even respond to this first strike in any meaningful manner. In addition to this, any vessel which is capable of traversing the interstellar void is going to be built of tough stuff in order to resist the rigours of exposure to hard radiation of varying types, meteor collisions and extremes of temperature etc, so shooting their ships down with ground based missiles would probably be more of an annoyance than any kind of serious threat, especially so if the ships hulls are bolstered by some kind of force field or energy dissipating material.That said, if you hit something hard enough and often enough in the right place, then it will eventually break, so all those experimental military spec railguns/coilguns that are out there would probably be useful for that.

Depending on what the aliens want from us means that a fair few options are open to them in terms of the actual military assault:-

Conventional "Combined arms"/"Total war" Assault. - As we would with our own conventional wars, the assault would begin with bombardment of targets of importance, taking out military barracks, airfields, missile silos, C3i installations and such, before aircraft move down from orbit in order to establish air superiority over the areas designated for landing zones.
Once these areas are cleared, landing vessels containing troops and ground vehicles would fortify the areas until permanent bases could be set up.

High/low orbit bombardment - If the aliens are following a "scorched earth" type agenda then wasting valuable resources and time on a difficult and costly ground war could easily be offset by blasting the surface of Earth with missiles and such without even landing, thus killing the humans en masse and driving the survivors away from the bits where they want to land/occupy.

Nuclear/Biological/Chemical warfare - A virus tailored to be deadly to humans could easily be whipped up by a sufficiently advanced race, and then disseminated into the air by unmanned drones. Similarly, poisoning the seas, rivers and reservoirs would lead to mass deaths and a complete breakdown of society within a matter of weeks, then all the aliens have to do is come on down and there will be many ready built cities for them to occupy and abundant resources to just pick up and carry away.

Mind control/Subliminal hypnosis - Using the Earths many orbital satellites, a subliminal signal is inserted into TV, radio and internet broadcasts, which eventually makes some humans more open to suggestion, or in extreme cases, completely brainwashed, so that when the aliens finally do come down and take over, people are not only pleased, they're actually happy.


So, what would our chances be?

Well, there is the distinct possibility that should E.T come a calling, their invasion would end up being completely destroyed en route by some sort of celestial phenomena, bad planning or even infighting amongst the invasion force (especially so if their journey is a long one).
Should they happen to get here though, bearing in mind that an alien race is able to construct vessels which can stand the rigours of the interstellar medium are gonna have to be, by necessity, well built and well protected, so slamming a couple of patriot missiles into their hulls more than likely isn't going to cause much by way of worrying hull damage. Also, it has to be bared in mind that a race sufficiently advanced enough to achieve interstellar travel isn't going to be stupid, so their offensive and defensive capabilities should be sufficiently advanced enough for their weapons to be more powerful and their armour to be resistant against most expected forms of attack, plus you'd expect that prior to wasting their time and resources on coming here to make war they'd at least do their homework to the point at which they were fairly confident of their chances of success against the yokels. It has to be said though that like us, this confidence, in whole or in part, could be purely psychological in nature. Imagine the look on trooper glurglzorgs face as he prepares to kill the first human of the war with his deadly poison gas grenade gun only to find that to the humans, the deadly gas actually induces fits of laughter or even makes the humans stronger and more able to kill him, having no adverse effects whatsoever against the enemy.

As we dont currently have access to death rays, laser guns, intelligent war robots or any other number of stereotypical sci-fi weapons, we will have to make do with what we got.
A machine gun may seem primitive compared to an alien wielding some sort of plasma bolt launcher, but if our own wars against each other have shown, if you hit something hard enough with a heavy fast moving object, eventually it will break, as the most advanced and powerful gun in the universe isn't going to help you if someone is busy caving your skull in with a hammer, or poking holes in you with a sharp stick, so, in conventional ground based warfare, I'd still say the odds were pretty even. If the aliens did use some kind of poisonous gas or chemicals, we have had protective suits to guard against that kind of thing for decades, and if we can build bunkers that can withstand 50 years of radioactive fallout, then short of completely destroying the planet there will always be survivors who are willing to fight back.

Either way though, win or lose, I reckons that soon after the alien war has finished, mankind will find a way to start making war on themselves again.