They were always fighting massive wars, and frankly while they may have held a lot of territory their influence on the total world wasn't there. Heck, they didn't even know about half the world's existence. The US controls the world without doing much in terms of constant large scale warfare. Just little crap like Afghanistan and Iraq. Though if large scale warfare were needed the US has easily the best military in the world, by far. I'm not sure if the next 10 most powerful armies in the world combined touch the United States. With the Navy alone, there are like 31 aircraft carriers in the world, and the US has 19 of them, and all 19 of them operating them are much better than any other countries (the countries that typically have 0-1 carrier). And not just by warfare, but in terms of economic and cultural power as well. In fact the idea of owning a bunch of land just doesn't work in terms of being a power in the world today. If the US really wanted to it would own pretty much all the Americas (both North and South), but occupying all those countries is not worth it. Having pretty much complete influence over them is much better, and we do, other than minor countries like Cuba.
Now I personally think we spend way too much money on the military, and that if we spent even half of what we spend now we'd still be for more dominant than everyone else...but we do spend a ton of money on it, and that's one of the reason the US is the greatest power the world has ever seen. It is the only country ever that can project its power anywhere in the world extremely quickly (perhaps the Soviets could at their height), due to its massive carrier fleet (and massive nuclear submarine fleet and its massive bombers and air force and everything else).
