Nuclear ICBMs already push us past that boundary. The world can no longer afford to fight a war seriously.
In that case you just nuke the shit out of everybody or create army if autonomous suicide bomber with nukes, biological and chemical weapons of all kinds. Once all humans are extinct the harmony on earth will be restored and everyone will leave happily ever after.
i'm not sure robot soldier is scarier than nukes. generally speaking, if they are just single task robots performing functions in dangerous situations, that seems like an improvement to risking human lives.
At some point, a military is going to develop autonomous weapons that are vastly superior to human beings on the battle field, with no risk of losing human lives, and there is going to be a blitzkrieg sort of situation as the relative power of nations shifts dramatically.
If we have two such countries we could have massive drone and cyberwars being fought faster than people even can comprehend what's happening.
Right now most countries insist on maintaining human control over the machinery of death. But that will only last for as long as autonomous death machines don't dominate the battlefield.
It's a fun challenge right now to build a machine that can win in Starcraft, but it's really a hop skip and a jump from there to winning actual wars.