This is the second time since 2007 that the military has determined it does not have a large enough force. Gates had already increased the size of the Army and Marine Corps shortly after taking the Pentagon job.
Gates noted that while progress in Iraq will lead to a reduction in the number of troops there, more troops are needed in Afghanistan because of the worsening violence in that conflict. He said the persistent pace of operations in the two wars over several years has meant a steady increase in the number of troops who are wounded, stressed or otherwise unable to deploy with their units.
Gates' announcement came after news thatby a roadside bomb in eastern Afghanistan Monday.
Also causing a shortage is the decision earlier this year to stop the unpopular practice of keeping troops beyond their enlistment dates, a practice known as "stop-loss."
"The cumulative effect of these factors is that the army faces a period where its ability to continue to deploy combat units (with enough troops) is at risk," Gates said.
"This is a temporary challenge that will peak in the coming year and abate over the course of the next three years," Gates told a news conference.
The Army currently has a total troop strength of 547,000.