Five of the seven Army National Guard brigades that were alerted Friday for probable deployments to Iraq in 2008 and 2009 already have done combat tours there, Army officials said. Of the seven, two will be used as combat brigades, replacing active-duty combat brigades now in the war. They are the 30th Heavy Brigade Combat Team, which served in Iraq from February 2004 to February 2005, and the 56th Stryker Brigade of the Pennsylvania Guard's 28th Infantry Division, which will be making its first deployment to Iraq as a Stryker brigade. The Stryker is a versatile combat vehicle newly deployed first used in combat in Iraq. Besides those two, four brigades will be used as security forces, helping to protect US bases, and one brigade will run prisons. Taken together, the National Guard deployments are intended as a one-for-one replacement of forces already in Iraq. Thus they do not represent a planned increase in the total size of the force, which currently includes 20 combat brigades and is scheduled to shrink to 15 combat brigades by July 2008.