Dave Truesdale pontificates about alien invasions in his latest “Off on a Tangent” column for F&SF:

Alien invasion has been one of SF’s most enduring themes. It has taken many literary variations and forms since H. G. Wells wrote The War of the Worlds in 1894, exposing our vulnerability and scaring everyone to death. While the human race survived his Martian invasion, it was due not to anything we as human beings did–after all, our military was powerless against the invaders–but through the inability of the Martian life forms to assimilate safely our planetary microbes, which turned out to be deadly to their alien immune system. If not for this oversight on the part of the Martians we would have been dead meat. Their overwhelming force and singular desire to crush us like bugs would have sealed our fate, no questions asked. They weren’t interested in negotiation or compromise, or enslaving us for whatever malevolent purpose most suited their alien intellect. They wished simply to exterminate us and take over earth for their own purposes. And there was nothing we could do about it. We were helpless. Toast.

Go read it and then argue about it.