The US stopped having wars with nations and instead has gone onto wars against ideas and things. The war against terror and the war against drugs being two of them. This way they can pick and choose which country to war with those elements without warring with a nation.
While I'm not a big fan of drones and other shadow warfare type approaches, it's not like the U.S. just decided this on its own. The terrorists are choosing the kind of warfare because they know they can't win a standup shooting war with the U.S.
The drug war is kind of a completely different thing as well because so much of it is a fight against our own citizens, and "victory" is pretty much unobtainable, which is maybe how the SWAT-team industrial complex wants it.