Indonesian forces consisted of 20,000 soldiers from the newly formed Tentara Keamanan Rakyat from its East Java Regional Command and an estimated 100,000–120,000 irregulars and militias.

When the battle reached its peak, the British sent additional troops which consisted of 24,000 fully armed soldiers from the 5th Indian Division, 24 Sherman tanks, 24 armed aircraft, 2 cruisers and 3 destroyers. Initially British troops were 6,000-strong lightly armed Indian soldiers from the 49th Infantry Brigade of the 23rd Indian Division.

An apparent misunderstanding about the agreement between British troops in Jakarta (lead by Lieutenant General Sir Philip Christison) and Mallaby’s troops in Surabaya was to have serious ramifications. Mallaby reached an agreement with Mr Suryo, the Republic of Indonesia’s governor of East Java, that the British would not ask Indonesian troops/militia to hand over their weapons.