Republican Senator John McCain has formally launched his candidacy for the 2008 presidential race.
McCain lost out on the Republican nomination in 2000
The 70-year-old Arizona senator and Vietnam veteran stressed his experience as he outlined his plan to rebuild the US military and strengthen the nation.
"I'm not the youngest candidate. But I am the most experienced," he said.
Mr McCain's announcement may bring new momentum to his campaign. He has faced criticism for his support for President George W Bush's new strategy in Iraq.
Mr McCain lost out on the Republican nomination to President George W Bush in the 2000 election.
This time round he faces stiff competition in the Republican camp from former governor of Massachusetts Mitt Romney and former New York mayor Rudi Giuliani.
Mr McCain, formerly seen as the strongest contender for the Republican nomination, has found himself trailing Mr Giuliani in first-quarter fundraising and opinion polls.
Making his announcement near a naval base in Portsmouth, New Hampshire, Mr McCain said he was not running for the White House "to be somebody" but to do his best for his country.
He emphasised his experience in both the military, as a former navy pilot, and as a four-term senator in Washington.
"I know how the military works, what it can do, what it can do better, and what it should not do," he said.
"I know how Congress works, and how to make it work for the country and not just the re-election of its members."
Mr McCain said the US government must "rethink and rebuild" its military and intelligence services, improve alliances with other nations and strengthen its diplomatic efforts.
He acknowledged that errors had been made in Iraq, but said the US must learn from its mistakes and complete its mission there.
"In the many mistakes we've made in this war, a few lessons have become clear," Mr McCain said.
"America should never undertake a war unless we are prepared to do everything necessary to succeed, unless we have a realistic and comprehensive plan for success, and unless all relevant agencies of government are committed to that success."
Mr McCain also called on Congress to put partisan interests aside and concentrate on finding solutions to the problems facing America.
Thorn in side
The BBC's Justin Webb in Washington says Mr McCain is a candidate who matters - he is nationally known and has been capable, almost single-handed, of standing up to the Bush White House.
But, our correspondent says, he is 70 years old and his campaign has not caught fire. He has been a thorn in the side of the Bush administration but has also recently become the foremost backer of the Iraq war.
If his candidacy fails, it will be enormously significant for the Republicans and potentially for America and the world, leaving the choice of candidate for the party entirely open and highly unpredictable, our correspondent says.
It could be that Mr McCain's support for the Iraq war assists him in getting the Republican nomination, our correspondent adds, but then destroys the Republicans' chances of winning the 2008 election.