WASHINGTON — New infusions of U.S. Marines will begin moving into Afghanistan almost as soon as President Barack Obama announces a redrawn battle strategy, a plan widely expected to include more than 30,000 additional U.S. forces.

Obama will try to sell a skeptical public on his bigger, costlier war plan by coupling the large new troop infusion with an emphasis on stepped-up training for Afghan forces that he says will allow the U.S. to leave.

Obama formally ends a 92-day review of the war in Afghanistan tonight with a nationally broadcast address in which he will lay out his revamped strategy from the U.S. Military Academy at West Point, N.Y. He began rolling out his decision Sunday night, informing key administration officials, military advisers and foreign allies in a series of private meetings and phone calls that stretched into Monday.

Military officials said at least one group of Marines is expected to deploy within two or three weeks of Obama's announcement, and would be in Afghanistan by Christmas. Larger deployments wouldn't be able to follow until early in 2010.

The initial infusion is a recognition by the administration that something tangible needs to happen quickly, officials said. The quick addition of Marines would provide badly needed reinforcements to those fighting against Taliban gains in the southern Helmand province, and could lend reassurance to both Afghans and a war-weary U.S. public.

The war escalation includes sending 30,000 to 35,000 more American forces into Afghanistan in a graduated deployment over the next year, on top of the 71,000 already there. Obama's announcement is the culmination of more than three months of debate over whether and how to expand U.S. military involvement in a war that has turned worse this year despite Obama's previous infusion of 21,000 forces.

But the numbers of fresh troops don't tell the whole story, White House press secretary Robert Gibbs said today. "It's what their mission is," he told ABC's "Good Morning America." "We're going to accelerate going after al-Qaida and its extremist allies. We'll accelerate the training of an Afghan national security force, a police and an army."

Obama also will deliver a deeper explanation of why the U.S. must continue to fight more than eight years after the war's start, emphasizing that Afghan security forces need more time, more schooling and more U.S. combat backup to be up to the job on their own. He will make tougher demands on the governments of Pakistan and, especially, Afghanistan, and will provide a fresh path toward disengagement.

Gibbs also promised that Obama would lay out an endgame scenario for U.S. involvement. "We want to — as quickly as possible — transition the security of the Afghan people over to those national security forces in Afghanistan," he said today. "This can't be nation-building. It can't be an open-ended forever commitment."

With U.S. casualties in Afghanistan sharply increasing and little sign of progress, the war Obama once liked to call one "of necessity," not choice, has grown less popular with the public and within his own Democratic party. In recent days, leading Democrats have talked of setting tough conditions on deeper U.S. involvement, or even staging outright opposition.

The displeasure on both sides of the aisle was likely to be on display when congressional hearings on Obama's strategy get under way later in the week on Capitol Hill.