I just saw it (I'm a little late ). If the earth's microbes killed them, then do they not have microbes on their planet? Or were ours different from ours and they had no immunities against them, and if so wouldn't they bring their own microbes that we would not be immune to (like small pox when the spanish arived in the Americas)?
Second, if they had been planning if for millions of years why wouldn't they think of the microbes? They burried the tripods and everything and it didn't occur to them that the earth's germs could hurt them?
Third, why would they have to send down their own people to pilate the tripods anyway? If they have such good technology why can't they just have them on some kind of "remote control"?