He said he would have called the troops home, not sent more to Afghanistan, if he did not think the mission in Afghanistan was important enough to risk American lives.
"You've done your duty -- not just when it's easy," Obama said. "That's why you've inspired your fellow Americans. That's why you inspire me. That's why you've earned your place next to the very greatest of American generations."
After a lengthy strategy review last fall, Obama announced in December that he was ordering a troop surge in Afghanistan to target insurgents and secure key areas here.
"As commander-in-chief, I have determined that it is in our vital national interest to send an additional 30,000 U.S. troops to Afghanistan," the president said at the U.S. Military Academy at West Point on Dec. 1.
"These are the resources that we need to seize the initiative while building the Afghan capacity that can allow for a responsible transition of our forces out of Afghanistan," he said.
The addition of 30,000 troops would bring the U.S. force in Afghanistan to about 98,000 troops, Gen. Douglas Lute, deputy national security advisor for Iraq and Afghanistan, said today.
There are currently about 80,000 Americans serving in Afghanistan, and the full surge force is expected to be in place by the end of the year.
In addition, there are 40,000 to 50,000 U.S.-allied coalition troops in Afghanistan.
In his December speech, Obama delivered a sober assessment of the security situation in Afghanistan and said the nation had "moved backwards" over the last several years because of the Taliban gaining momentum.
Knowing the concerns of the American people about getting into another open-ended conflict, Obama said that this troop surge would have a deadline -- "After 18 months, our troops will begin to come home," he said -- but said it is necessary to establish conditions for an accelerated transfer of security responsibility to Afghan forces.
"Just as we have done in Iraq, we will execute this transition responsibly, taking into account conditions on the ground," the president said.
The Obama administration laid out a very tight schedule for deployment and withdrawal. American troops began arriving in Afghanistan early this year -- "the fastest pace possible," the president said in December -- and the expectation is that they could begin to come home starting in July 2011.
An ABC News-Washington Post poll in January found that 50 percent of Americans approved of the president's handling of the situation in Afghanistan, down from 63 percent approval in April, while 45 percent of Americans disapproved.
According to a report by The Associated Press, the number of U.S. troops killed in Afghanistan has roughly doubled in the first three months of this year compared to the same period in 2009. There has also been a dramatic spike in the number of wounded, with injuries more than tripling in the first two months of 2010 and the latest data for March suggesting a continuing trend.
In his December speech, Obama outlined how the Taliban and al Qaeda have created instability in Afghanistan, but he also noted that the elected government there has been "hampered by corruption, the drug trade, an under-developed economy, and insufficient security forces."
Obama delivered a blunt, tough message to Karzai and his government: "The days of providing a blank check are over."