The U.S. Army has decided not to reduce the tour of duty in places like Afghanistan and Iraq, from twelve months to six or seven months. There were two main reasons for not having shorter tours, similar to those used by the other services. First, the army units are the most heavily involved in reconstruction and peacekeeping. This involves direct contact with locals and it takes time to build relationships. New units coming in take a month or so to get tight with the local relationships the units before them had built up, and are passing on. So if you cut the tour to six months, you have less time in country with all your local connections tight. 

The second problem has to do with the special training units go through before they are sent overseas. This would be the same for a twelve or six month tour. Actually, National Guard and reserve units go through more training than active duty troops, and would suffer even more from the shorter tours.

While the marines do a lot of the same work as the army overseas, marine training and overseas deployments has long been based on shorter tours. The army could adapt to the marine system, but that would take a lot of time and effort. Moreover, the marines still pay the price of having to spend all that time getting connected with locals when they arrive. In fact, some marine units, like headquarters (that have a lot of the officers who maintain contacts with the locals), do twelve month tours in Iraq. The army could afford to provide shorter tours for troops who just do non-combat support work, and don't deal a lot with the locals or require special training. But this would be bad for the morale of the combat and combat support units doing the longer tours.