I think the simple truth is that people don't want to fight in wars they don't believe in. If the Russians invaded Alaska or Canada tomorrow, there would be a million volunteers by Sunday ready to die for god and country. If Iran invaded Israel tomorrow, they'd have to reinstate the draft, and spend billions tracking down the endless dodgers.
Its a simple lack of motivation and incentives.
It's not a lack of the willing. Outside of war or times of provocation the biggest lure to the military is as training and a career.
People enlist for mostly practical reasons.
There are still enough willing. What's lacking is the willing and capable.
The future Yurpian and Murican defenders of the free world!
As much as I think obesity is a big problem in the US, I would think the rise of the economic climate has also taken a toll on available recruitment. With more jobs around, the military would become a less desirable option instead of "the only way out". Especially for healthy, smart individuals that have observed the news (ongoing iraq/afghanistan wars.)
From my knowledge in the Netherlands we have much less obesity, but Dutch Military and especially armed forces still have a very hard time to get enough staff. Special forces is passing grades 6 out of 10 nowadays while standard was 8-9 out of 10 grades. High brass gave as main reasoning that there are to many career opportunities which are much safer, require no travel and pay much better while also offering future incentives. Due to the fact that we had mandatory 12 month conscription for all eligible males from 1946 untill1996 the military has not been really popular as a career choice either.