I talk about this topic often, ts, I think assassinations are the way to go, government sponsored I mean, in the place of wars...at least in certain situations...
Kings don't kill kings.
Besides, everything is a two-way street. Does that mean countries can assassinate Obama? It's much easier to assassinate the U.S. president than to take on the U.S. in war. Your world order would pretty much look like New York mafia wars.