If you're looking for a deeper explanation that goes beyond the "they're all pussies now" kind, read Arnold Toynbee's A Study of History. (Or read the D. C. Somervell abridgement...it's a lot shorter.) Toynbee was amazingly prescient, and if you adjust for a few factors, his cyclical view of history is rolling along as predicted for the U.S. and the West in general.
Where his view needs some adjustment is in two areas: Today, the largest nations can project their militaries anywhere on the globe at the push of a button. This alters how cultures behave at their boundaries. Now that our world is fully divvied up, borders don't shift like the tides. Secondly, with the internet and global media, ideas spread at light speed around the planet. So good ideas and bad alike spread very quickly. But America and the West are a visionless bunch to whom the rest of the world no longer looks for leadership. And we're clearly in decline.