With the war over, the Dutch government sought to regain control over its former colony in Indonesia, which had declared itself independent from the Netherlands on 17 August 1945. On 26 October, British soldiers and men from the Netherlands Indies Civil Administration landed in Surabaya to occupy the city, which had recently been evacuated by the Japanese garrison. This act caused tensions to flair between the British and the local Indonesians, many of whom now considered themselves to be citizens of an independent Indonesia. Violence soon began, and soon former members of PETA, including Mangundiprojo, formed units to fight against the British forces. A ceasefire was called on 29 October, and local resistance leader Oerip Soemohardjo selected Mangundiprojo to work with the British to ensure the ceasefire was maintained. While touring the city with British brigadier Aubertin Walter Sothern Mallaby, the British officer attempted to secure the safe return of a group of Indian soldiers whom had been surrounded by a crowd of independence supporters. The situation escalated when Mangundiprojo was briefly captured by the Indian soldiers, and in the confusion Mallaby was killed by a member of the crowd while the brigadier sheltered in his car. His death provoked a reprisal by the British, starting the battle of Battle of Surabaya. Mangundiprojo fought during the battle against the British. During the later Indonesian war of Independence, Mangundiprojo played a role in seizing a large sum of money that the Dutch colonial authorities were keeping in a bank.