WEST POINT, N.Y. -- Declaring "our security is at stake," President Barack Obama ordered an additional 30,000 U.S. troops into the long war in Afghanistan Tuesday night, nearly tripling the force he inherited as commander in chief. He promised an impatient public he would begin bringing units home in 18 months.The buildup to about 100,000 troops will begin almost immediately -- the first Marines will be in place by Christmas -- and will cost $30 billion for the first year alone. In a prime-time speech at the U.S. Military Academy, the president told the nation his new policy was designed to "bring this war to a successful conclusion," though he made no mention of defeating Taliban insurgents or capturing al-Qaida terrorist leader Osama bin Laden. "We must deny al-Qaida a safe haven," Obama said in spelling out 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 practiced by al-Qaida." "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.