Two years ago, the U.S. Army was ordered by Congress to expand its personnel strength 13.5 percent (from 482,000 to 547,000) within five years. The army said this would cost $70 billion, but Congress did not provide all that money. While the army was able to recruit the additional troops, they didn't really want them. Some politicians are urging the army to expand by another 30,000 troops. All this is propelled by political and media anxiety over the short period of time troops were home ("dwell time") in between overseas assignments. In 2007, at the height of the Iraq fighting, army troops were spending a bit less than a year at home for every year overseas. Now the dwell time is 17 months for every 12 months abroad. The problem is gone, but some politicians are still trying to make something out of it.
Two years ago, the army was also in the midst of a reorganization, which didn't change the number of troops, or equipment, in a brigade, but does change how they are organized and used. The reorganization created more brigades, and made the army even better able to deal with the kind of heavy deployments required in 2005-7.
The math works like this. The army, marines and reserves can muster about sixty combat brigades. During 2004-7, there were 19 brigades deployed to combat zones (15 in Iraq, three in Afghanistan and one in South Korea.) That's when the army began working to get active duty troops two years dwell time for every year in a combat zone. For reserves, the goal was home for four years, overseas for one. With help from the marines, the army can just about make that.
The most critical reasons behind the two year dwell time goal were morale, keeping combat veterans in uniform, and the reduction of combat fatigue. The more you keep the troops in a combat zone, beyond a certain number of months, the less likely they are to re-enlist. Note that everyone in the army works on employment contracts (of 3-4 years, usually). Not everyone renews their contracts when they expire. But since September 11, 2001, an above average number of people have. This is very important, because people (officer or enlisted) who "re-up" are the most valuable people you can have. They are experienced, many of them "combat experienced." But keep them out there too long, and they will start to leave. Not in large numbers.
The U.S. Navy has had the same problem, because of the long deployments at sea sailors often had to endure. That experience enabled them to work out a formula, which calculated the number of sailors they would lose, for a taskforce, for each additional day, beyond the usual six months, they kept them at sea. The army is about to encounter a similar effect. The army is not publicizing their anticipated losses (people who don't re-enlist), but was apparently up to several thousand troops a year. That doesn't break the army, but does provide more headaches for those in charge of recruiting and retention. The senior generals treat this sort of thing as "losses." Not combat losses, the people who don't re-enlist leave the army in one piece.
The more time you spend in combat, without dwell time, the more likely you are to develop combat fatigue. That can mean anything from transferring to a non-combat job, to a medical discharge. Both of those options cost the army money.
The army is already losing several billion dollars a year to maintain the additional 65,000 troops Congress forced them to add to total strength. That's money the army doesn't have to take better care of the troops in general. Another 30,000 troops just makes it worse, especially with the army budget now shrinking. It's unlikely that there will be another war like Iraq anytime soon. Afghanistan will never require as many troops as Iraq, simply because landlocked Afghanistan does not allow you to get sufficient supplies into the country to support more troops.