Isn't that illegal? Or is that legal, because there is officially war between American and Yemen? (serious)

I thought the legal implications of terminating people from a foreign country remotely is either an act of aggression or already war, depending on how the affected political party sees it.

So either it means:

a) America would have to pay for the losses and families.

b) Yemen and America are in war.

c) Yemen capitulated against America.

Otherwise I don't understand. :-/

(Sorry, if someone feels that this is too logical, non-emotional. I'm of course, against killing and drone attacks.)