Contrary to what the Beeb would have you believe...
the US is not an aggressive empire building regime. It is much more profitable to compete economically. Also, just because you cannot measure the cost in lives does not mean that there are no costs.
Interesting that you should compare this development to that of the atomic bomb. Look carefully at history and see if you can figure out the number of lives that have been saved by the fear of the use of that monster. WWII cost millions of lives, and that style of warfare was accepted practice until the advent of the Bomb. Would Korea have sparked WWIII without the threat of mutually assured destruction hanging over the heads of the US and USSR? Would it have been the same with Vietnam? How about any of the minor little skirmishes around the globe since the end of WWII? The bomb however, managed to keep everyones head on straight through the cold war, and put a lot of incentive behind those that would find a peaceful solution to problems. In those days, a failure of the UN could cost more than sanctions. It was an ugly thing to be avoided, we have two examples of the outcome of failure to prevent use of the Bomb.
Now, as for Automated weaponry. We are not developing Cylons here. At least, not yet. It is hard enough to prosecute a war where the enemy hides among the civilian population and fights more through propaganda than through traditional combat with human guidance and control. We are far from turning over that sort of control to the machines, Captain Cyborg and malicious cyberloos not withstanding.