His absence was mocked by his rivals, who tried to fill the space
vacated by Mr Trump's boycott by attacking each other on immigration and
other issues.
Mr Trump decided to withdraw after Fox News refused to drop debate host Megyn Kelly, whom he accused of bias.
The billionaire held a rally nearby, in honour of war veterans, that threatened to overshadow the debate itself.
On Monday, voters in Iowa are due to pick their presidential nominee for each party.
Days
ahead of that critical test, Mr 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 humour in the opening minutes by throwing mock insults at his rivals.
