Story Highlights• NEW: McCain says he wishes he had described loss of troops as 'sacrifice'
• Obama says McCain wasn't denigrating troops; loyalty not in question
• McCain announced that he would run in '08 on Wednesday's David Letterman
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WASHINGTON (CNN) -- Republican Sen. John McCain on Thursday became the second presidential contender to back away from saying the war in Iraq had "wasted" the lives of U.S. troops.
During a Wednesday night appearance on CBS' "Late Show with David Letterman," McCain announced that he would enter the 2008 race but said a formal announcement would come in April.
He also said the nation had paid a "grievous price" for mistakes made in the first years of the nearly four-year-old war in Iraq.
Americans "are very frustrated," he said, "and they have every right to be. We've wasted a lot of our most precious treasure, which is American lives, over there."
More than 3,100 U.S. troops have been killed in Iraq since the March 2003 invasion that toppled dictator Saddam Hussein.
The Arizona senator has been harshly critical of the Bush administration's management of the war, but he is a prominent advocate of President Bush's deployment of more than 21,000 additional combat troops.
The Democratic National Committee called for an apology. However, in a written statement McCain said only that "I should have used the word 'sacrificed,' as I have in the past."
He added that U.S. leaders owe the armed forces "our best judgment and honest appraisal of the progress of the war."
"With a new commanding general and a new strategy, we are now trying to correct those mistakes, and I believe we have a realistic chance to succeed," he added.
"That does not change the fact, however, that we have made many mistakes in the past, and we have paid a grievous price for those mistakes in the lives of the men and women who have died to protect our interests in Iraq and defend the rest of us from the even greater threat we would face if we are defeated there."
His statement followed similar back-tracking by Illinois Sen. Barack Obama, a Democratic presidential contender. In February, Obama told an Iowa audience that "we now have spent $400 billion and have seen over 3,000 lives of the bravest young Americans wasted."
Obama, who has called for U.S. combat troops to leave Iraq by next March, later called the remark "a slip of the tongue." But while his party's leaders called for an apology from McCain, Obama came to his defense Thursday.
"I don't think he can be questioned in his dedication to American troops," Obama told reporters.
"We have a duty -- a sacred duty -- to make sure we are honoring their sacrifice by giving them missions in which they can succeed," he said. "I'm positive that was the intent in which he meant it. It was the same intent that I had when I made my statement."
McCain was a Navy aviator during the Vietnam War and spent more than five years in a North Vietnamese prison camp after his jet was shot down.
Obama said that while he and McCain may disagree, "The one area that I don't think he can be questioned [on] is his dedication to American troops. He's been there. He's done that."
CNN's Dana Bash and Ted Barrett contributed to this report.