Washington (CNN) -- President Obama will announce the U.S. troop strategy for Afghanistan in a speech at 8 p.m. ET Tuesday at the U.S. Military Academy at West Point, New York, White House spokesman Robert Gibbs said Wednesday.



In the speech, Obama will explain why the United States is in Afghanistan, its interests there and his decision-making process, Gibbs said, but "the president does not see this as an open-ended engagement.



"Our time there will be limited, and I think that's important for people to understand," he said.



"We are in year nine" in Afghanistan, Gibbs told reporters. "We're not going to be there another eight or nine years."



Obama will meet with members of Congress at the White House on Tuesday afternoon before the speech.



Cost issues are among the topics the president will address, Gibbs said.



"It's a million dollars a troop for a year," he said. "Ten thousand troops is $10 billion. That's in addition to what we already spend in Afghanistan and Pakistan. That also does not include training, and it doesn't include the maintaining of a security force. It's very, very, very expensive."



But, Gibbs added, "I think the president, throughout this process, has talked about the cost in terms of American lives and in terms of the cost to our treasury, and I think he'll continue to talk about it."



The president ordered more than 20,000 additional troops to Afghanistan in March. Gen. Stanley McChrystal, the U.S. commander in Afghanistan, reportedly has called for up to 40,000 more to wage a counterinsurgency campaign against the Taliban, the Islamic militia originally ousted by U.S. military action in 2001.



Obama has weighed several options for bolstering the American contingent, ranging from sending a few thousand troops to the 40,000 McChrystal requested.



A defense official told CNN earlier this week the Pentagon is making detailed plans to send about 34,000 more troops to Afghanistan in anticipation of Obama's decision on the 8-year-old war.



There had been no final word on Obama's decision as of Tuesday, said the Defense Department official, who has direct knowledge of the process. But the official said planners have been tasked with preparing to send 34,000 additional American troops to Afghanistan with the expectation that Obama was leaning toward approving that many.



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Defense Secretary Robert Gates is expected to receive the paperwork Thursday to approve orders to deploy 1,000 Marines from Camp Lejeune, North Carolina, to Afghanistan in late December -- the first of the new troops to be sent, a U.S. military official told CNN on Wednesday. The official is not authorized to talk about the plans because they have not been officially announced.



The 1,000 Marines are part of a battalion task force that has been preparing and training for the deployment, the official said.



The president held a lengthy meeting with top advisers Monday night, and he told reporters Tuesday that he would announce new plans for Afghanistan after Thanksgiving.



McChrystal took part in the meeting Monday, along with Vice President Joe Biden, Gates, Joint Chiefs of Staff Chairman Adm. Michael Mullen and Karl Eikenberry, the U.S. ambassador in Kabul, Afghanistan.



Obama said Tuesday that the deliberations had been "comprehensive and extremely useful."



"It's going to be important to recognize that in order for us to succeed there [in Afghanistan], you've got to have a comprehensive strategy that includes civilian and diplomatic efforts," he said at a news conference Tuesday with visiting Indian Prime Minister Manmohan Singh.



The defense official said Tuesday that the military is planning to send three U.S. Army brigades, totaling about 15,000 troops; a Marine brigade with about 8,000 troops; a headquarters element of about 7,000; and between 4,000 and 5,000 support troops -- a total of about 34,000 troops. CNN reported last month that this was the Pentagon's preferred option.



However, the official said Wednesday that the Marines are expected to deploy 8,000 combat, aviation and support troops, plus 1,000
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the 8-year-old war. 1,000 President Obama why the United States is in Afghanistan, its interests there and his decision-making process, why the United States is in Afghanistan, late December
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