President Barack Obama Tuesday evening announced a commitment of 30,000 additional troops to the fight in Afghanistan for a limited mission that he said will see them begin to return home in 18 months after they have weakened — but not necessarily defeated — the Taliban.

Obama, who spoke at the United States Military Academy in West Point, N.Y., declared that the U.S. commitment to Afghanistan is not open-ended. He said the objective of the new troops will be to deny a safe haven to al-Qaida and to create the conditions for Afghans themselves to battle the Taliban.

“We must deny al-Qaida a safe-haven,— Obama said. “We must reverse the Taliban’s momentum and deny it the ability to overthrow the government. And we must strengthen the capacity of Afghanistan Security Forces and government, so that they can take lead responsibility for Afghanistan’s future.

The president described the circumscribed mission as not only adequate to combat the threat, but as the kind of effort the United States can afford. Obama justified the linkage of cost to the conflict by equating U.S. economic security with the U.S. national security.

“Our prosperity provides a foundation for our power,— Obama said. “It pays for our military. It underwrites our diplomacy. It taps the potential of our people, and allows investment in new industry,— he continued. “That is why our troop commitment in Afghanistan cannot be open-ended — because the nation that I am most interested in building is our own.—

Obama also indicated that aid to Afghanistan will be conditioned on the performance of the country’s leaders in rooting out corruption. “The days of providing a blank check are over,— he said.

In a brief interview with Roll Call Tuesday just before the speech, White House Senior Adviser David Axelrod promised Obama would be “actively— engaged in seeking to convince Congress to support his Afghanistan strategy.

“I think he’s going to make his case to the Congress and the American people,— Axelrod said. “This is an issue of vital national importance.—

Obama in his remarks sought to pre-empt criticism expected from both the right and the left in Congress. He indicated that beginning a withdrawal now was a nonstarter.

“To abandon this area now and to rely only on efforts against al-Qaida from a distance would significantly hamper our ability to keep the pressure on al-Qaida and create an unacceptable risk of additional attacks on our homeland and our allies,— Obama said.

The president rejected suggestions from the left that he was leading the United States into “another Vietnam,— arguing that the U.S. this time has allies in the conflict, was itself attacked from plotters in the war zone and does not face an indigenous insurgency as the United States once did from the Viet Cong.

Obama suggested that maintaining the status quo — and not adding the additional troops — would in fact delay the ultimate withdrawal of troops. “It would ultimately prove more costly and prolong our stay in Afghanistan, because we would never be able to generate the conditions needed to train Afghan Security Forces and give them the space to take over,— he said.

Anticipating GOP objections to his setting of any kind of timeline, Obama said the partial one he has set — which includes a date for beginning a withdrawal but no suggestion about how long it will take — was needed to prod the Afghans to begin to take responsibility for their own security.