When the Axis powers invaded Yugoslavia in April 1941, Lukačević became a leader of Chetniks in the Sandžak region and joined the movement of Draža Mihailović.

While the Chetniks were an anti-Axis movement in their long-range goals and did engage in marginal resistance activities for limited periods, they also pursued almost throughout the war a tactical or selective collaboration with the occupation authorities against the Yugoslav Partisans.

In January and February 1943, while under the overall command of Major Pavle Đurišić, Captain Lukačević and his Chetniks participated in several massacres of the Muslim population of Bosnia, Herzegovina and the Sandžak.

Immediately after this, Lukačević and his Chetniks participated in one of the largest Axis anti-Partisan operations of the war, Case White, where they fought alongside Italian, German and Croatian (NDH) troops.

After returning to Yugoslavia in mid-1944, and in anticipation of an Allied landing on the Yugoslav coast, he decided to break with Mihailović and fight the Germans, but this was short-lived, as he was captured by the Partisans a few months later.

[3] 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.

[13] In early February 1943, during their advance northwest into Herzegovina in preparation for their involvement in Case White, the combined Chetnik force massacred large numbers of the Muslim population in the targeted areas.

[19] Lukačević and his Chetniks were drawn into closer collaboration with the Axis during the second phase of Case White,[20][21] which took place in the Neretva and Rama river valleys in late February 1943[22] and was one of the largest anti-Partisan offensives of the war.

[23] Despite the fact that the Chetniks were an anti-Axis movement in their long-range goals[24] and did engage in marginal resistance activities for limited periods,[25] their involvement in Case White is one of the most significant examples of their tactical or selective collaboration with the Axis occupation forces.

[28] In February 1943, during the second phase of Case White, Lukačević and his Chetniks jointly held the town of Konjic on the Neretva river alongside Italian troops.

[29] After being reinforced by German and NDH troops and some additional Chetniks, the combined force held the town against concerted attacks by the Partisans over a seven-day period.

[36] In November, Seitz and another American liaison officer, Captain Walter R. Mansfield, conducted a tour of inspection of Chetnik areas, including that of Lukačević.

[40] Under the agreement, a special German liaison officer was assigned to Lukačević to advise on tactics, ensure cooperation, and facilitate arms and ammunition supply.

[43] Also during December, the Higher SS and Police Leader in the Sandžak, SS-Standartenführer Karl von Krempler, posted notices authorising local Serbs to join Lukačević's Chetniks.

On 22 December, shortly after the conclusion of Operation Kugelblitz, Oberst (Colonel) Josef Remold issued an order of the day commending Lukačević for his enthusiasm in fighting the Partisans in the Sandžak, and allowed him to keep some of the arms he had captured.

[45] Lukačević and the others were detained by the British in Bari and thoroughly searched by local authorities, who suspected them of a robbery that had occurred in the Yugoslav consulate in Cairo a short time before.

[46] In mid-1944, after Mihailović was removed from his post as Minister of the Army, Navy and Air Force as a result of the dismissal of the Purić government by King Peter,[47] Lukačević attempted to independently contact the Allies in Italy in the hope of "reaching an understanding on a common fight against the enemy".

[49] On 19 October, Lukačević proposed that the Chetniks change their policy to greet the Red Army as liberators and ask to be taken under the command of a Russian general.

[49] Subsequently, he deployed his 4,500 Chetniks into southern Herzegovina and for several days from 22 September they attacked the 369th (Croatian) Infantry Division and the Trebinje–Dubrovnik railway line, capturing some villages and taking hundreds of prisoners.

[52] However, the Partisans, concerned Lukačević was trying to link up with a feared British landing on the Adriatic coast, attacked his forces on 25 September, first capturing his stronghold at Bileća and then comprehensively defeating him.

[52] With several hundred remaining Chetniks, Lukačević withdrew as far as Foča before returning to the Bileća area in the hope of linking up with small detachments of British troops that had been landed to support Partisan operations.

He was accused of conducting the massacre at Foča, participating in the extermination of the Muslim population, collaboration with the occupying forces and the Serbian puppet government of General Milan Nedić and the commission of crimes against the Partisans.