With more troops going to Afghanistan over the next six months, and lots of troops not coming back from Iraq (without being replaced) until later next year, the U.S. Army will not be able to give troops all the time at home it wants them to have. This time at their home base, with their families, is called "dwell time," and the more of it you have between combat tours, the less likely you are to have stress related mental problems.
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. 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 did 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. It was believed that, with a little help from the marines, the army can just about make that. The increase in troops sent to Afghanistan will delay this dwell time plan for a few years. The Department of Defense has announced that there will be higher numbers of U.S. troops in Afghanistan for 2-4 years. At least that's the plan. And after the plan, the soldiers will have their dwell time.
There are several reasons for the two year dwell time goal. These include 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. That has gone up even more in the last year, because of the recessing.
Keeping combat veterans in the army is very important, because people (officer or enlisted) who "re-up" are the most valuable people you can have in combat. But keep them out there too long, and they will start to leave. Not in large numbers at first, but eventually you will suffer large losses.
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 (that gets you a pension and life-time medical care). Both of those options cost the army money. The army would rather see if additional dwell time will enable troops to recover from the stress the comes from being in a combat zone (even if not in combat). The additional dwell time would be possible if most troops were withdrawn from Iraq. That's not a sure thing until the Iraqis settle some of their current disputes.