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!!

It is important to keep in mind how much technology is needed for interstellar travel. If we wanted to badly enough, we would be capable of interstellar travel with our current technology. The main technological hurdle is not any propulsion technology but perfecting the technique of maintaining a self-contained artificial ecosystem for an extended period of time-probably with a nuclear reactor providing power to lights that would maintain photosynthesis. The simplest and most reliable propulsion technology would be a solar sail, basically a huge sheet of aluminized mylar reinforced with composite fibers. It would look a lot like a parachute, but with a canopy multiple kilometers across and the ship being towed behind it. The acceleration produced is fairly weak, but it doesn't require reaction mass or energy from the ship to be generated for thrust. The ship can be turned around as it approached its destination and the solar sail would become a solar parachute decelerating the ship as it approached the new star. Depending on the length of the journey the ship might host multiple generations of passengers. Various types of nuclear fired rockets are the next most likely type of propulsion, but face the hurdle of requiring an enormous amount of energy and reaction mass to power them. Everything else is basically Star Trek-style fantasy at this point. However our understanding of physics and especially quantum physics is in its infancy.  

Ok , this alien race finally received the broadcast transmission of Mars Attacks and it pissed them off. Say their homeworld was two to three times the size of Earth. Would their evolution in a heavier gravity allow them to pick up say an M1 and toss it like a horseshoe?

Also, how long would it take us to adapt some of their weapons for our own use. If it was an invasion or say subjigation rather, they would have to send ground forces and there will be instances where we will be able to get our hands on some of their gear. As in the previous example of the roman army versus a modern army, there will be instances where the romans get some of the rifles and such.