Since September 11, 2001, the U.S. Army has had to keep a large number of its troops overseas most of the time. In the last two years, some twelve percent of the army has spent 400 or more days away from their home base. That's 55 percent of the time, and for some it's been over 70 percent. Another 310,000 have spent 220-400 days away. That's 30-55 percent of their time. Only about a quarter of the army's troops have not been deployed overseas in the past two years. So far, people have been joining, and staying in, at above average rates. But the brass know their history. In the past, Americans have tended to grow impatient after about three years of war. This includes World War II. Knowing that the large number of troops serving in remote areas (without their families) will eventually result in morale and retention (keeping career troops in) problems, the generals are looking for ways to avoid the morale meltdown, or at least spread the pain sufficiently so that there is not a massive loss of veteran troops. 

The U.S. Army has always had "unaccompanied (by family) tours." But for half a century this mainly meant Korea and a few small locations, which usually worked out to less than ten percent of the army doing a "hardship tour" at any one time. Now you have three times as many troops doing the "hardship" duty and some of them will decide to leave the service if they keep getting that kind of work year after year. Worse yet, a disproportionate number of those doing back-to-back hardship tours are in jobs that already have a shortage of people. This includes linguists, helicopter maintainers and other technical specialists. The army can ill afford to lose these specialists, as they are expensive to train and are already in short supply. But there's a war on and having these people in the combat zones in a matter of life and death. So more effort is going into recruiting and training the specialists, or getting capable people already in uniform to volunteer for retraining. If the army cannot prevent an exodus of key people, they will have to hire civilians to do these jobs. This is already happening to a limited extent. The army has less control over the civilians, although many are former military and are cooperative about the working conditions and dangers. But, technically, such civilians can just quit and grab the next plane home if they get shot at or otherwise frightened one time too many.