NEW YORK // The US president Barack Obama, in a defining moment of his presidency, announced he would deepen the US involvement in Afghanistan, sending 30,000 more troops to fight the Taliban despite Americans' growing pessimism about the war. Mr Obama tempered the build up by pledging that the first troops would return by July 2011 and stressing that Afghan forces would be rapidly trained to take over the fight. He also called for additional commitments from US allies and pledged to strengthen ties with Pakistan, where al Qa'eda and Taliban fighters have been based. The troop build up will begin almost immediately - the first Marines will be in place this month - and will cost $30 billion (Dh110bn) for the first year alone. Following previous troop increases, it will almost triple the force Mr Obama inherited on taking office in January. "I do not make this decision lightly," he said. "I make this decision because I am convinced that our security is at stake in Afghanistan and Pakistan." The long-awaited, nationally televised speech at the US Military Academy at West Point was a political gamble for Mr Obama, whose popularity has fallen as he struggles to win support for a health care overhaul and deal with rising unemployment and a soaring budget deficit. Mr Obama won the Democratic Party's nomination partly because of his steadfast opposition to the Iraq war - a conflict he took pains to distinguish from the fight in Afghanistan. In pledging to pull some troops out in July 2011, Mr Obama did not say how many would be left and for how long. War opponents fear the United States will be dragged into an endless conflict that divides the nation, undermines the economy and wrecks a presidency that began with huge expectations.