WASHINGTON (CNN) -- The deployment of 21,500 additional U.S. troops to Iraq, including five Army Brigades, will be accomplished without breaking a promise to soldiers to give them at least one year back home between deployments, Pentagon officials told CNN Wednesday.

However, the president's plan will require one National Guard brigade, scheduled to return to the United States in March, to remain in Baghdad four extra months, until mid-summer. Sources identified that brigade as the 1st of the 34th Division of the Minnesota National Guard.

The rest of the increase will be accomplished by what Pentagon officials described as a "modest acceleration" of previously scheduled deployments of Army brigades, which are made up of roughly 3,500 soldiers.

The first additional brigade into Iraq would be from the 82nd airborne Division, which is already in Kuwait and could be in Baghdad by week's end, officials said. The next brigade would be the 4th of the 1st Infantry Division based in Fort Riley, Kan., which would deploy to Iraq about two weeks earlier than originally scheduled.

That would be followed in the coming months by Army brigades from Fort Lewis, Wash., and Fort Stewart and Fort Benning in Georgia.