The Haitian president Jean-Bertrand Aristide fled the country at the weekend, following a dispute over who should be in charge there.
Some people accused him of getting into power through unfair elections. Rebel forces challenged his leadership.
Widespread unrest broke out, but now a group of US marines has arrived in the country to act as peacekeepers.
They will be followed by a larger force, including French and Canadian troops.
UN Secretary General Kofi Annan said the world had not forgotten the people of Haiti.
"We understand their need and are standing by them in their hour of need," he said.
Meanwhile, Mr Aristide has travelled to Africa, where he is expected to seek asylum.