Prior to Operation Barbarossa in late June 1941, the OUN actively cooperated with Nazi Germany. According to the National Academy of Sciences of Ukraine and other sources, OUN-B leader Stepan Bandera held meetings with the heads of Germany's intelligence, regarding the formation of "Nachtigall" and "Roland" Battalions. On 25 February 1941, the head of the Abwehr, Wilhelm Franz Canaris, sanctioned the creation of the "Ukrainian Legion" under German command. The unit would have had 800 persons. Shukhevych became a commander of the Legion from the OUN-B side...

In spring 1941 the legion was reorganized into three units. One of the units became known as Nachtigall Battalion, a second became the Roland Battalion, and a third was immediately dispatched into the Soviet Union to sabotage the Red Army's rear...

It is estimated that in June–July 1941 over 4,000 Jews were murdered in pogroms in Lviv and other cities in Western Ukraine. There is controversy, though, regarding the extent and scope of the participation of the Nachtigall Battalion and Roman Shukhevych in these atrocities, as well as in the Massacre of Lviv professors...

Nevertheless, records show that the Nachtigall Battalion subsequently took part in the mass shootings of Jews near Vinnytsia in July 1941.