In World War II, many resistance groups from different nationalities throughout Europe fought the Nazis. Among them were Jewish groups, who fought either on their own or as part of other national groups. Such groups began to operate in Eastern Europe as early as 1941, mostly in the forests in White Russia, Poland, and Lithuania. Later, they were absorbed by the Russian partisans on the Soviet side, while in Poland they continued to operate on their own. In all, there were more than 20,000 Jewish partisans, many of whom showed great courage and resourcefulness in fighting under difficult conditions and with scant arms against the German war machine. The song of the Vilna partisans, Shir Ha-partisanim, is sung every year on Yom Ha-Shoah.