Nazi Germany's invading forces entered Šiauliai on 26 June 1941. The next day, Greimas met with other partisans and was put in charge of a platoon. He handed down an order from the German Commandant to round up 100 Jews to sweep the streets. He felt uncomfortable and did not return the next day. Nevertheless, he became an editor of the weekly "Tėvynė", which urged ethnic cleansing of Jews from Lithuania. The nominal editor, Vladas Pauža, was a proponent of genocide. In 1942, in Kaunas, Greimas became active in the underground Lithuanian Freedom Fighters Union, which derived from the genocidal Lithuanian Nationalist Party, which the Nazis had banned in December 1941, after Lithuania's Jews had been killed. He grew close to life long liberal-minded friends Bronys Raila, Stasys Žakevičius-Žymantas, Jurgis Valiulis, all of whom had supported ethnic cleansing of Jews from Lithuania.