The M23 rebels are going to take back the areas that a regional force is leaving in eastern Congo.
About 300 soldiers from Kenya were the first to leave their posts in Goma, the capital of North Kivu province, early on Sunday.
About a year ago, soldiers were sent to the region because the government of DR Congo asked for help. They wanted to stop the M23 rebel group from becoming strong again.
However, the government has said that the East African force is not doing a good job and they have decided not to continue supporting it.
Kenya soldiers are now in control of Kibumba, Kibati, and other areas near Goma.
The M23 said it will take back all the areas it gave to the regional forces at the start of the peace process.
A truce that was promised last month has been ignored, and now there is more fighting.
The President of DR Congo, Félix Tshisekedi, wants to be President again. He said areas controlled by rebels might not be able to vote in December because they are not safe.
It is not known when soldiers from other countries will begin to move away from their positions.
Last year, M23 rebels gave back the areas they had taken over in North Kivu to soldiers from Burundi, Kenya, South Sudan, and Uganda.
This happened because of an agreement for peace made in discussions in Kenya and Angola.