Candidates, journalists and politicians have been quick to pass judgment on the remarkable outcome in the Iowa caucus.
AP Photo, Charles Dharapak
Mitt Romney won the poll by just eight votes ahead of Rick Santorum.
It was the first in a series of ballots to take place in every US state to decide the Republican candidate for this year's presidential election.
Speaking as the final votes were being counted, former Massachusetts governor Romney congratulated Santorum but added: "We think it has been a great victory for us here too... On to New Hampshire. Let's get the job done."
He later tweeted:
"Thank you, Iowa! What better place than the heartland of America to start the restoration of America's heart and soul."
Rick Santorum, a former US senator, responded to the outcome with the declaration: "Game on." He later tweeted:
"Thanks to all of you, we pulled off our #iowasurprise! Keep us going to NH, SC & beyond."
NH refers to New Hampshire, the scene of the next ballot on 10 January. SC refers to South Carolina, which holds its poll on 21 January.
Texas congressman Ron Paul, who came third in the Iowa caucus, has also vowed to stay in the race. "This momentum is going to continue," he told a crowd of supporters. "We will go on, we will raise the money." A spokesperson for Paul tweeted:
"The campaign is celebrating a great victory. There were three tickets out of Iowa, and Ron Paul earned one of them."
Paul's national campaign chairman, Jesse Benton, said: "The Ron Paul campaign is celebrating a great victory tonight... This is now a two way race between establishment candidate Mitt Romney and the candidate for real change, Ron Paul."
Former House of Representatives Speaker Newt Gingrich finished fourth. "We are not going to go out and run nasty ads," he later said. "But I do reserve the right to tell the truth. And if the truth seems negative that may be more of a comment on his record than it is about politics."
Texas governor Rick Perry, who was the frontrunner back in August, came fifth in the poll. He announced he was suspending his campaign and returning to his home state. He said:
"With a little prayer and reflection, I will decide the best way forward."
Barack Obama, the sitting Democratic president, is not competing in any caucuses or primaries this year. He is seeking a second term in office unopposed.
However the head of his re-election campaign, David Axelrod, had this to say on the Iowa result on Twitter:
"The sound Santorum is hearing right now is not the buzz of victory. It's the whirring of Romney super political action committee, preparing to carpet bomb him."
Jeff Zeleny, writing in the New York Times, believes that despite the ultra-close result, Romney is the long-term winner. "The Iowa caucuses, which sounded the opening bell of the Republican contest, did not bring the clarity to the nominating fight as Mr Romney had hoped," Zeleny argues.
"But even though he did not secure the authoritative victory that he had fought for in the last week, he handily dispatched two rivals who were once seen as his biggest threats, Mr Perry and former House Speaker Newt Gingrich."
The BBC's North America editor Mark Mardell agrees. "In the end this result has to be good for Romney," he says. "He's achieved this in a state he hadn't bothered to fight until the last few weeks, where he didn't have much of a campaign.
"But it also underscores his problem. He can't break away from the pack. There is deep uncertainty about him among conservative voters, and a complete lack of passion or enthusiasm for him. What happens if he never has a clear-cut, open-and-shut victory?"
But historian Tim Stanley, writing for the Telegraph, says Romney shouldn't celebrate too soon. "He won with only 25% of the vote in a heavily divided field," Stanley notes.
"75% of Iowans rejected a man that many commentators consider to be the only electable Republican in the running."
In contrast, he adds, "the conservative movement has a new star: former Pennsylvania senator Rick Santorum."
Meanwhile for the Spectator, Fraser Nelson reflects on what this all means for the current US president. "Sometimes the American political system can pick unlikely winners, as happened with Obama," he says. "At other times it can end in spectacular and farcical indecision, like Florida 11 years ago. Iowa seems to be going through one of these less decisive moments."
"Lucky old Obama."