Rebel forces have handed control of the port in Liberia's capital, Monrovia, to West African peacekeepers.
The rebels have not managed to stop the looting
Although much of the food in the port's warehouses has been looted during two months of fighting, deliveries of badly-needed food, water and medicines can now be stepped up.
After weeks of conditions, deadlines and delays, the first wave of some 200 US Marines arrived at Liberia's international airport to back up the peacekeeping operation.
Screaming jets from the warships which have dominated the horizon since President Charles Taylor went into exile on Monday circled the capital, as tens of thousands of people took to the streets, cheering.
"This operation today is going to be an important one," American ambassador John Blaney said at the airport.
"You are going to see American boots on the ground, and a firm commitment to uphold humanitarian concerns in this country."
BBC correspondent in Liberia Alastair Leithead says the strong show of force will help persuade the rebels to pull out of Monrovia.
At the port, the rebel commander Sekou Fofana shook hands with the American ambassador before retreating.
"We have no reason to doubt the credibility of the Americans, and we have no reason to doubt the credibility of the peacekeepers, so we will leave as we have said. I'm leaving right now," Mr Fofana said.
The rebels are expected to move out to the River Po on the city's outskirts.
Liberians have been calling for the US to intervene since the fighting reached Monrovia in June.
"I am so happy. All these years we've been praying for America to come," said Randolph Eggley, a 51-year-old worker at the airport. "Today maybe peace will begin."
But chaos in the port area continued, as rebel fighters fired guns in a vain attempt to stop thousands of people looting food from warehouses.
Our correspondent says there is now very little food left.
Blah in Ghana
Although some 2,300 US troops are based on ships off the Liberian coast, the US insists that the main peacekeeping role in Liberia should be played by Ecomil, which currently consists of less than 800 Nigerian troops.
A battalion of Nigerian
peacekeepers arrived at Sokoto airfield in the north of the country, ready to be airlifted to join Ecomil.
Meanwhile, new interim President Moses Blah has begun talks with Liberian rebel leaders in neighbouring Ghana.
Ghana's Foreign Minister Addo Akufo Addo told AFP news agency an agreement could be signed as early as Saturday.
The rebels' presence has stopped humanitarian aid reaching hundreds of thousands of famished refugees in Monrovia.
An emergency relief co-ordinator in Liberia, Carolyn McAskie, said the World Food Programme had a supply ship anchored offshore, as well as food aid in the region ready to be flown in.
But she warned that more peacekeepers were needed.
Another battalion of Nigerian troops is due to arrive in Monrovia in the coming days and the Pentagon said the aim was for the US reaction force to stay in Liberia for a short a time as possible.
Mr Taylor flew into exile in Nigeria on Monday as part of attempts to end years of conflict.
Interim President Blah - Mr Taylor's former deputy - is set to rule Liberia until October.
Lurd rebels have rejected his appointment but say they will respect the ceasefire.