Everyone makes fun of Signs because it depicts an alien invasion that is thwarted because the aliens die if they touch water. But that's not the issue. It's not their fault that water is like acid to them, but it is their fault that they showed up naked. You have intergalactic starships, but you don't have goddamned pants? How does a civilization's evolution just skip over that part?
It's like humans landing on a planet where 70 percent of the surface is covered in molten lava, and the inhabitants are basically just moving sacks of lava. Even the atmosphere is so dense with lava vapor that often lava just rains from the sky with little to no warning. So what's your plan of attack? If you say anything other than "Jump out of the spaceship completely naked, your junk proudly flopping about, and engage the lava monsters in hand-to-hand combat," then congratulations -- you are smarter than the aliens in Signs.
The invasion in War of the Worlds seems better thought out, initially. Their shields are impervious to bombs and bullets, and the human defense strategy is apparently to ignore this and keep shooting until the aliens fall down out of pity. So with the human race on its knees, what do the aliens do to celebrate? Run around naked, drink dirty puddle-water and put everything they can find in their mouths like a bunch of unsupervised toddlers. Days later, every single one of them is dead from the common cold.