Former government soldiers and those from two main rebel groups in the Democratic Republic of Congo have formed a united force.
There are concerns for the militias who fought in the war
The ceremony to mark the merger took place in Kisangani in the north-east.
The soldiers from the government, Congolese Rally for Democracy (RCD) and Congolese Liberation Movement (MLC) will be deployed in eastern DR Congo.
Early this year, the country's warring parties signed a power-sharing deal to end five years of fighting.
The BBC's Arnaud Zajtman says the merger has been welcomed by local people in Kisangani, who witnessed the ceremony alongside the DR Congo and Belgian defence ministers.
He said the fact that something concrete was happening on the ground was a sign of hope, though concerns remain about how young militias who fought alongside rebels and government troops during the five-year war will be treated during the military integration.
It is not clear whether all the soldiers who fought in the war will be absorbed into the united army.
DR Congo Defence Minister Jean Pierre Ondekane, from the RCD, told the first batch of 600 soldiers to observe strict discipline while in the force.
The new army will be trained by officers from the Belgian army, before they are deployed to Bunia and other parts of DR Congo, where UN peacekeeping troops are based.