Mrthfx

I want to create a scenario where highly technological aliens invade a medieval earth, the kind with magic and dragons and such. But I don't want the aliens to be completely overpowered. I want them to be scary and come out of nowhere but give the humans a chance to win the war. I think the best way to do this is not giving the aliens guns, or explosives, or plasma weapons. These aliens need to have only melee weapons. the aliens would still have far better means of transportation, communication, and armor, but the humans have magic and dragons to counter this. It's a perfectly balanced war.

I just need to know if and why such an advanced race would have such primal weapons...

What if they did bring bucket loads of weapons to try to invade us, but the environment on earth rendered them incapable of working effectively?. Maybe the high amount of nitrogen in our atmosphere or even moisture (water vapor).

This would be possible if your alien race was a hot headed bunch who did not do their homework first before invading :P. They then have to resort to primitive weaponry like swords and clubs just to defend themselves.

What you seem to be describing is some form of 'gap' in their technology tree, assuming the same development path as we have taken.

For us, we consider electro-magnetic technology to be almost child's play so as to be so simple, and a massive amount of our modern technology is actually based on this. But, the only reason that we think that is because of the Maxwell equations of 1861-1862. Prior to that, if someone like Queen Victoria had said 'you engineers, build me a box I can speak into that makes my voice come out in front of all my subjects', it simply couldn't be done because we didn't have the scientific theories available to design a practical application like radio.

We've already made some scientific progress integrating weak nuclear forces with electromagnetism, and we expect the next one will be strong nuclear, then gravity. Once we reach that point, we have a grand unified theory that could give us incredible control over the universe and our place in it. But, that's assuming there even is a GUT to be discovered.

The answer to your question however seems to be in the power sources for your alien ships. If your aliens have mastered theories around Strong Nuclear forces, then they may well be able to initiate fusion reactions, giving them vast amounts of power for spaceships, but not have even basic radio. Also, based on what we know, it's possible that fusion doesn't scale down or miniaturise well, meaning that it's possible (especially if their planet doesn't have the ingredients for gunpowder readily available near the surface) that your aliens could have fast ships, but only basic weapons.

Ultimately, what we think of as the 'obvious' technology tree is really a function of what we've discovered through our own scientific endeavours. But, it's entirely possible that other beings, having discovered other principles of the universe first, could get here and then go 'damn! How come they have such powerful weapons and haven't even mastered the basics of space flight?'

Can't numbers be a balancing factor? Let's say the spaceship holds a couple thousand fighters, and there are at least millions of humans. That could work well even without magic and dragons. The humans have bows and crossbows, siege weapons (even the romans had catapults and ballistas), so it would be weird if the aliens didn't have any of those (or couldn't copy any of those in a day).

A fairly simple one but it could just be a matter of culture for the aliens. As a highly honour based society they wouldn't dream of anything less than taking their enemies on in personal combat, seeing ranged weaponry as weak of character and cowardly.