The U.S. is just about finished with its military operation to liberate Iraq from decades of dictatorship. The Sunni Arab minority did not give up power without a long fight, an effort that reached its peak in 2007, when most Sunni Arab Iraqis turned against the Islamic radicals leading the fight against democracy. At its peak, the U.S. had 171,000 troops in Iraq. And that was a struggle. That's because, in the 1990s, the U.S. cut military personnel by a third. The Cold War was over and, it was thought, so was the prospect of major military efforts.
Thus in 2003, the U.S. ground forces consisted of 512,000 troops in the army and 178,000 in the marines. The army also had 550,000 troops in the reserves and National Guard, but only a fraction of these could be on active duty at any given time. As a practical matter, this provided the army with another 170,000 active duty troops. The marines only had 47,000 reservists, and these were equal to another 15,000 active duty troops. So the U.S. had about 880,000 ground troops to sustain a combat force overseas. Iraq was not the only overseas combat zone. There were over 70,000 troops operating in trouble spots all over the world (Afghanistan and South Korea being the only ones to get much media attention.) Then there was the problem that not everyone in the army, or even the marines, had a job that was needed in a combat zone. Moreover, at any time, about 20 percent of the troops were training or in some kind of school. Troops also get 30 days of vacation a year, which takes eight percent of force off duty at any given time. Although the army and marines tried really hard to make sure everyone went "down range" at least once, there were still a lot of military jobs that were in big demand in the combat zone (like Special Forces, intelligence, bomb disposal and communications.) The navy and air force helped out by sending support troops, with the needed skills, but it still wasn't enough.
The army and marines quickly rediscovered what they had learned in World War I and II; combat troops, and even support troops, wear out pretty quick in a combat zone. It's the stress. Even Iraq and Afghanistan were not a unique experience. During the Vietnam war, it was noted that troops who did two tours (13 months each) with only a year off in between, tended to get stressed out. It was something of a cliché among the troops, but to the medical professionals, the pattern was clear.
In Vietnam, a lot of U.S. troops didn't show up until 1965, when the force there went from 23,310 at the end of 1964 to 184,000 at the end of 1965. It kept increasing (385,000, 485,000) until it peaked at 536,000 at the end of 1968. Then the force shrank each year (484,000, 336,000, 158,000) until, at the end of 1972, there were only 24,000 trainers and advisers there. In Iraq, an invasion force of 150,000 was there for most of 2003, then it shrank for a year or so, before growing again and peaking at 171,000 in late 2007. That's when the Sunni Arab terrorists were crushed (but not completely eliminated). That left the enemy weak enough for the new Iraqi security forces to deal with. So the U.S. force began shrinking (to 142,000 in early 2009, 98,000 in early 2010 and 50,000 in August 2010).
There were far fewer troops in Iraq than in Vietnam (1.1 million versus 2.7 million), and even fewer combat deaths (4,500 versus 57,000). The army was twice as large during the 1960s, but that was not enough to sustain the larger numbers in Vietnam. The big lesson learned in the last half century is that the longer troops are in a combat zone, the more stress they endure and the less capable they become. Political leaders tend to be oblivious of this problem, and military leaders do not consider it a top priority. But the stress issue has forced its way to the top of the queue. This should not be a surprise.
In past centuries, similar attrition was noted within armies that spent a lot of time at war. This is often attributed to disease, poor sanitation and inadequate food and water supplies. But from the 18th century forward, many more troops knew how to write and left written accounts of what really went on. And that was that a lot of troops would get stressed out from too much campaigning and just desert. That was a lot easier to do in times past, and many troops just walked away. Bullets and bombs may appear to be the big danger, but less explosive factors (disease and stress) are what does the most damage.