The U.S. Army is experimenting with Regionally Aligned Brigades (RABs). These are regular combat brigades that customize their training to prepare for likely service in a particular part of the world. The first brigade to be so trained (from the 1st Infantry Division in Fort Riley, Kansas) will be prepped for service in Africa. That means the brigade will be getting ready to work for AfriCom (Africa Command). That could include everything from sending teams to train troops, to joint training with African troops, to peacekeeping or aiding in some catastrophe. If this experiment succeeds the army will designate RABs for Southern (SouthCom in South America), Central (CentCom in the Middle East), and Pacific Commands (PacCom). RABs would be assigned to commands in numbers each of the commands feels is adequate.
This form of specialization has been going on for decades in the U.S. Army Special Forces. Troops in the seven Special Forces groups tend to spend their career in one group, the better to become experts in languages and culture in the regions each group specializes in.
The 1st Special Forces Group specializes in Southeast Asia, Korea, China, and the Pacific in general. The 3rd Special Forces Group covers the Caribbean and West Africa. The 5th Special Forces Group specializes in the Middle East and South Asia (including Afghanistan and Pakistan). The 7th Special Forces Group specializes in Latin America. The 10th Special Forces Group specializes in Europe (especially the Balkans). There are two groups in the reserves. The 19th Special Forces Group covers the same territory as the 1st and 5th Groups. The 20th Special Forces Group covers the same territory as the 7th Group.
Special Forces have suggested several times in the past that the army try something like RABs and the U.S. Marine Corps picked up on the idea years before the army actually did.