After the outbreak of World War II, the government of Regent Prince Paul of Yugoslavia declared its neutrality. Despite this, and with the aim of securing his southern flank for the pending attack on the Soviet Union, Adolf Hitler began placing heavy pressure on the Kingdom of Yugoslavia to sign the Tripartite Pact and join the Axis. After some delay, the Yugoslav government conditionally signed the Pact on 25 March 1941. Two days later a bloodless coup d'état deposed Prince Paul and declared 17-year-old Prince Peter II of Yugoslavia of age. Following the subsequent German-led invasion of Yugoslavia and the Yugoslav capitulation 11 days later, Lukačević went into hiding in the forests. He soon returned to Belgrade, where he became aware of the activities of Draža Mihailović. He then left the capital with some other officers and soldiers to form a Chetnik detachment in the Novi Pazar area of the Sandžak region. On 16 November 1941 Muslim forces from Novi Pazar and Albanian forces from Kosovo attacked Raška and quickly advanced toward the town. They were commanded by Aćif Hadžiahmetović. The situation for the defenders became very difficult, so Lukačević personally engaged himself in the defence of the town. On 17 November they stopped the advance of Hadžiahmetović's forces and forced them to retreat. On 21 November Lukačević took part in the attack of Chetnik forces on Novi Pazar. In the summer of 1942, Lukačević and his Chetniks fought the Partisans in Herzegovina.