Declaring "our security is at stake," President Barack Obama ordered an additional 30,000 U.S. troops into the long war in Afghanistan on Dec. 1, but balanced the buildup with a pledge to an impatient nation to begin withdrawing American forces in 18 months.

Barack Obama AP PHOTO

In a prime-time speech at the U.S. Military Academy, the president said his new policy was designed to "bring this war to a successful conclusion." The troop buildup will begin almost immediately — the first Marines will be in place by Christmas — and will cost $30 billion for the first year alone.

"We must deny al-Qaida a safe haven," Obama said in articulating U.S. military goals for a war that has dragged on for eight years. "We must reverse the Taliban's momentum. ... And we must strengthen the capacity of Afghanistan's security forces and government."

The president said the additional forces would be deployed at "the fastest pace possible so that they can target the insurgency and secure key population centers."

Their destination: "the epicenter of the violent extremism."

"It is from here that we were attacked on 9/11, and it is from here that new attacks are being plotted as I speak," the president said.

It marked the second time in his young presidency that Obama has added to the American force in Afghanistan, where the Taliban has recently made significant advances. When he became president last January, there were roughly 34,000 troops on the ground; there now are 71,000.

In eight years of war, 849 Americans have been killed in Afghanistan, Pakistan and Uzbekistan, according to the Pentagon.