The 12 and 13 month tour dates back to the Korean war, when so many World War II veterans in the reserves were called up to fight in yet another war. This was considered unfair, as was the traditional policy of keeping troops in the combat zone for the duration (until the war was over.) So, as a political expedient to deal with a widely unpopular war, the 13 month tour was established. It was used once more in Vietnam.
The shorter air force tour is justified by the fact that air force personnel are just going to another air base to work with other air force people, doing the same thing they did back in the states. Except for an occasional mortar attack or ambush, that is. Most navy people are out in the Persian Gulf, here one patch of open water is pretty much like any other. So six months seems about right for being away from the comforts of land bases.
The army has another reason for the shorter tour, especially for combat troops. The action is pretty intense if you are infantry, armor or aviation. And the army knows from long experience that the longer you keep combat troops in continuous action, the more stress problems you have. Even though troops are given two weeks of vacation while in Iraq, making the tour of duty shorter is an attractive option, and will probably at least be tried by the army.
The U.S. Army is considering sending its troops to Iraq for shorter periods of time, so that the troops will last longer in combat. The U.S. troops sent to Iraq are all going there for different lengths of time. If youre in the army, you go there for 12 months, while air force people stay only four months. Most navy personnel are there for six months at a time. Marine combat troops go for seven months, although marine headquarters units stay for twelve months. The army long believed that the 12 or 13 month tour was necessary because of the need for troops to get to know their surroundings, and the people they have to deal with. But the marines, who do the same work as the army troops, only stay for seven months. The army has noted that the there is little difference in the performance of army and units, and is considering switching to the shorter marine type deployments. The army believes that this will help with recruiting, and keeping people who are career troops.