The battle started out between Malaysian forces and Sarawakese rebels. The poorly-armed Sarawakese rebels were not successful against the might of the Malaysian army. Soon, after Indonesia declared war on Malaysia, they expressed their support for Sarawakese militants, however they provided limited support. After two unsuccessful battles in the capital city of Kuching by the Sarawakese militants, the Indonesians decided to physically invade Sarawak. After the Indonesians entered Sarawak, they were met with welcome by the Sarawakese independence supporters. The Indonesians also helped properly arm the militants.
The Indonesians overran much of Malaysia's southern forces, capturing soldiers along the way. The Indonesians adopted a unique policy of causing no harm to the civilians, even if they try to use a weapon, so long as it doesn't kill. The Indonesian Army also dispatched Relief Brigades, providing food and shelter for displayed people in Sarawak.
It was one of the biggest victories achieved by the Indonesian forces during the war, they pushed the Malaysians up north and east of the Rajang River (the portion of Sarawak that remained part of Malaysia). The Battle of the Rajang River is considered by many to be what granted Sarawak its independence.