The New York billionaire decided to withdraw after Fox News refused to drop debate host Megyn Kelly, whom he accused of bias.
Donald Trump held a rally nearby – in honor of war veterans – that threatened to overshadow the debate itself.
On February 1, voters in Iowa are due to pick their presidential nominee for each party.
Days ahead of that critical test, Donald Trump’s absence on the stage in Des Moines was keenly felt by his seven rivals in the race to be Republican presidential nominee.
Texas Senator Ted Cruz addressed it with humor in the opening minutes by throwing mock insults at his rivals.
“I’m a maniac and everyone on this stage is stupid, fat and ugly, and Ben [Carson], you’re a terrible surgeon,” he said.
Former Florida Governor Jeb Bush also poked fun at Donald Trump – his chief tormenter in previous debates – by saying how much he missed him.
Elsewhere in Des Moines at the same time, Donald Trump led a raucous rally in honor of the country’s war veterans.
“When you’re treated badly, you have to stick up for your rights,” he said, referring to his row with Fox.
Fox News released a statement saying that Donald Trump offered to appear at the debate if the broadcaster contributed $5 million to his charities, but they refused.
Data released by Google after the debate suggested that search interest in Donald Trump still far surpassed the other candidates.
Many observers on social media thought the event was duller without Donald Trump.
Some of the night’s most heated moments were provided during exchanges about immigration.
Ted Cruz and Marco Rubio were both forced to explain video clips of previous statements that appeared to be at odds with their hard-line campaign pledges.
They then turned on each other, with Marco Rubio saying immigration was “the lie that Ted’s campaign is built upon”.
An Iraq war veteran who came to the US from Mexico as a child appeared via YouTube to tell the candidates that “some of the comments in this campaign make us question our place in this country”.
Jeb Bush applauded Dulce Candy and said “we should be a welcoming nation”.
The Iowa caucuses on February 1 are seen as the first real test of the election campaign, and the beginning of a series of state-by-state contests to choose delegates for both Republicans and Democrats.
Unlike a primary, which is a traditional election featuring secret ballots on polling day, the caucuses in Iowa are meetings of registered party voters and activists where they discuss the candidates and then vote.
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