Pentagon Behind Obama Schedule for Deploying Troops to Afghanistan

The Pentagon expects to deploy 18,000 of the 30,000 troops called up to Afghanistan by late spring, a slower pace than the White House envisioned, but necessary, say Pentagon officials, because President Obama did not want to shorten troops' rest time at home.

So far, 4,500 troops have recently deployed to Afghanistan to add to the 68,000 already on the ground.

Army soldiers alternate between one year at home and one year deployed. A senior Pentagon official speaking on background Tuesday said the the only way to get all 30,000 troops into the country by June 30, Obama's original goal, was to reduce the year at home for several soldiers.

"The 30,000 additional troops that I'm announcing tonight will deploy in the first part of 2010 -- the fastest possible pace -- so that they can target the insurgency and secure key population centers," Obama said in announcing his troop surge in December at West Point. "They'll increase our ability to train competent Afghan security forces, and to partner with them so that more Afghans can get into the fight. And they will help create the conditions for the United States to transfer responsibility to the Afghans."

However, the next day, Press Secretary Robert Gibbs lengthened the deadline to the end of the summer of 2010.

Needless to say, "dwell time" is a critical issue for soldier morale and the White House agreed to let soldiers have one year at home. That means the Pentagon is now on pace to get all 30,000 troops in by the end of September.

The official said the end-of-summer deadline hasn't changed.

Meanwhile, Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, told a Senate panel Tuesday that the next 12 to 18 months will be critical to reversing momentum gained by insurgents in Afghanistan

America's future security is "greatly imperiled if we do not win the wars we are in" because "the outcome of today's conflicts will shape the global security environment for decades to come," he said.

Fox News' Justin Fishel and Kelly Chernenkoff contributed to this report.