What led to or what leads to?

It depends on whether it is present or past tense. if you are saying, "what leads to World war?" you are asking a question about what currently leads to world war. But if you ask, "What led to world war?" then you are asking about a past world war and the causes leading up to it.

For #2 it would be what led to the invention of computers? because the other option is what lead to the invention of computers?, and the phrase what lead is grammatically incorrect.