I'm gonna go back on myself a little bit by saying that my worst case scenario example may not be the worst possible.

A friend put it forward to me that perhaps their could be an alien race who are completely invincible. I know it seems extreme, but if you think about it there is no reason why an alien race wouldn't be able to replace cells quicker than they die, and still maintain the same basic biological structure. Thus these aliens wouldn't die (of natural causes) and would be able to heal wounds unbelievably quick. Now I would say that these aliens would still have found ways to kill each other, because I know it is possible to kill something if you can destroy its cells quicker than it can replace them and it is incongruous to think an alien race would be unable to kill itself. My friend interrupts me here and suggests that these aliens have never had an internal war, and therefore have had no need to produce weapons. This would mean that these aliens may never have experienced death and because they could replicate their cells very quickly we would be unable to kill them with anything less than a nuclear (or really powerful conventional eplosive) bomb. I think that if an alien race hadn't experienced death at any point whatever, they would surely have no moral opinion on killing. This would mean that they probably wouldn't be aggresive, because they are not threatened in any way, and they probably woudn't have any intention of killing, because they've never heard of it.  Why Then would they invade?  Well if they regenerated their cells, that organic material had to come from somewhere. To regenerate that quickly they would need a massive enegy input, which would lead directly to a means of attack. Starvation. If they came, we'd have to slow them down until they either starved or moved on. Unfortunately the only way to starve an invinvcible race, would be to destroy all the organic material we couldn't hide, fortunately we have thousands of cubic miles of caves and tunnels across the world. So in the end we could defeat almost any alien race that invaded (please dont get invasion confused with annihalation), agree?

Aliens capable of FTL travel would probably have enough mechanization to deploy completely robotic forces to complete extermination with no risk to themselves.  Much of the evolution of US military equipment and practice in the last 30 years has been in lowering risk to the individual soldier/sailor/airman.  It would be much closer to 'Terminator' than 'War of the Worlds.'  Semi-autonomous or autonomous machines would be doing the work while the aliens orbited out of easy reach.  Our ability to exhaust their supply of resources, whether that meant fuel, food, equipment, would probably be greater than our ability to kill them outright.  

mex ftl travel requires a bit more than an advance in theory it
requires some fairly hefty rewriting of the laws of the universe as we
understand them today.  we can see some pretty funky stuff at the
quantum level but getting anything like that to happen at the macro
scale doesn't seem to be a happening thing.

Back when Newton first wrote out his Laws of  Motion, Kinetics, & Gravity,
no one believed in those days and ages that mankind would one day be flying heavier-than-air machines to cross the globe in a matter of hours just as routinely as people his day and age rode horse and carriage.

It took the ideas of two men named Bernoulli and Venturi, and years of inventors' ingenious tinkering, to prove that an airfoil shape could be created that allows us to, effectively, "skirt around" gravity.

 

Give some extraterrestrial culture a few hundred more years (or thousands?) of advancement beyond mankind's achievements,

and it shouldn't be all that hard to comprehend that, while they may not technically be able to travel faster than light, through ingenuity and further understandings of other physical laws (certainly all of which even we don't know), they may have found a means of "skirting around" the limitations imposed by light speed travel just as we, technically, circumvent the pull of gravity by using airfoils acting on the atmosphere.

I doubt any alien race would attack a home world unless they were dead set trying to commit genocide. And if they were they must really hate us and we would really hate them and it would most likely end up with huge casualties on both sides.

 

 If an alien race turned up and started mining mars, taking water from europa etc, setting military bases up on mercury etc, would we be able to defend our territory? Thats a diffrent matter.

 

 Would humans stand by and watch another species do extremely well off the resources in our solar system? Claiming planet after planet and taking all the valuables?

 

 No way. We would launch nukes, build spaceships and go nuts to protect the bodies around the sun. Could we sustain a war off earth? I think given the will we could.  We went from Wright brothers to Landing on the moon in less than 70 years, and that wasn't a flat out race and we are far more capable now than then.

 

 If we turned the entire world over to producing space weapons with in a few years we would be pretty competitive to fend off most attempts on our solar system.

 

 Much like the US/Russia in WWII. The world is a very good producer/manufacturer. The germans may have had the technological edge (ICBM's, jet planes, Uboats etc) but production is a killer. Unless they have planet size ships able to build competitively near here forget it.

or to be really sneaky,, cause a greenhouse effect,  or pop yellowstone with some funky space laser.

life is fairly fragile, and any invader wouldnt be trying to wipe us out, theyd have to be real smart to NOT wipe all the multicellular life out.

you put a big impactor or 2 in, global winter,

but that said what reason would they have for coming here? surely anything that major is gonna knacker up their war-aims?

id say your motive would have to be :

slaves

religion

 

nothing else would be worthwhile.

we'll use a 2 gigaton thermonuclear depth charge type weapon on the deep ocean trenches.

yellowstone and  any major calderas they get the laser.

oh, and we'll use the big laser to write on the moon, a cryptic message....'cha...'

 

Technological advance doesn't necessarily automatically make the more advanced force invulnerable to the weapons of the less advanced force. Sometimes it paradoxically makes the more advanced force more vulnerable to the older weapons. Take the Prince Eugen vs. Ticonderoga for example. One on One, with no supporting vessels.

 

On the open ocean the Ticonderoga clearly wins, mostly because it carries 2 helicopters, which can attack the Prinz Eugen without endangering the Ticonderoga, and because those helicopters carry radar that allows them to engage the Prinz Eugen at beyond visual range, making them invulnerable to the Prinz Eugen's Flak guns. Now let's consider moving the battle to Norway. The Prinz Eugen can hide in a fjord, forcing the Ticonderoga to Hunt for it, or more specifically forcing the Ticonderoga's Helicopters to hunt for it. If it can find a place where a fjord is relatively narrow and curves fairly sharply so that there isn't a long line of sight, it might be able to force the helicopters to come within range of it's heavy Flak guns before it is detected.

 

If it were able to dispatch the helicopters the fight would greatly change, especially if the battle took place in restricted waters, but even on the open ocean. The problem is that our antiship technology has become so effective, that we don't bother armoring our ships anymore. This has been a recurring theme in history. Once offensive technology reaches a certain point armor goes away until new armor technologies can be developed. The Ticonderoga's primary long range anti-ship weapon is 8 Harpoon anti-ship missile launchers. Each Harpoon has aproximately a 500 lb warhead. I'm not entirely sure how much armor the Prinz Eugen has, so I'm not entirely sure what the results of a Harpoon hit would be, but a 500 lb warhead is roughly equivalent to a 10 inch shell, and the Prinz Eugen was a Heavy Cruiser with 10.5 inch guns, so I would think that such hits would cause major damage, but I don't know if they would be sufficient to sink it. If it was able to survive, it would take a bit for the Prinz Eugen to find its range, but once it had done so, it would be hitting an unarmored ship with 10.5 and 8 inch shells. Ouch.

 

The same principle applies to land technology. Lets say you take the Romans against a WW2 era Infantry force. Clearly the WW2 infantry have a big advantage in ranged attack - guns vs slings and javelins. However if the Romans figure out that their massed infantry don't stand a chance and break their forces up, the picture improves. They sill have a fight on their hands, but a sling has some advantages over a gun, such as being silent. If the romans captured some hand grenades, I'd think that a sling would be excellent for the purposes of throwing grenades. If they were actually able to close to melee range, then it would be a WW2 soldier with a bayonet that they don't practice using all that much and no armor against a legionare with armor, shield, and short sword, and very very skillful in its use. The Romans would lose massive numbers of troops in the war, but assuming that they also have a much greater numbers of troops at their disposal and had the will to keep fighting they could win.

Treadgar

That's a good point the Prinz Eugen vs. a Tico. Imagine if a Zulu warrior gets close enough to some of our modern soldiers with that asagai (spelling?).

The Zulus got close at Isandhlawana, against the most modern soldiers of that time, and killed them to a man. When things get up close and personal, Assagai is to Rifle as Rock is to Scissors. 
 Would the thin steel or aluminum hull of a modern frigate/destroyer stand up to the point blank fire of Nelson's time?

 

Once again, IF Earth was attacked by an Alien race that had the technology to actually harness inter-steller space travel then there is NO NEED for the Aliens a) to attack in the first place, but for the sake of this hypothetical, b ) there would be no need for the Aliens to actually land here on Earth (and the 100m army, et cetera is hence taken out of the equation), they would simply be able to pick off all of our major military installations using weapons we simply would not understand and wouldn't realise that we were attacked until it was literally too late; and dont even bother with nukes; intelligence is the number one game in war and I'm pretty sure that an Alien race would have done their homework previously and would dispose of all of our nukes before they even had the opportunity to be launched, and if by some miracle some were launched then they would be swatted out of the sky like flies before they could detonate close enough to harm their ships...

 

This is not a Zulu versus Modern British soldier scenario, this is a contemporary US Airforce versus Cavemen and that's being nice to the cavemen!!