For the USSR, Volhynia had been part of the Ukrainian SSR since November 1939. On 11 July 1943 UPA units, together with local peasants, attacked nearly 100 Polish villages and settlements; it was the bloodiest day of the Volhynian massacre, the campaign by the OUN-B and UPA to murder and drive out the Polish population. On the night of 3–4 January 1944 the Red Army crossed the pre-war Polish border here, and the Home Army's 27th Volhynian Infantry Division fought both the Germans and the UPA.
1919
1939
- 1 September – 6 October 1939BattleInvasion of PolandFrom 1 September to 6 October 1939 Poland fought Nazi Germany, Slovakia and, from 17 September, the Soviet Union. The campaign ended in partition.
- 17 September – 6 October 1939BattleSoviet invasion of PolandFrom 17 September 1939 the Red Army occupied eastern Poland, resisted by the KOP, Grodno, Lwów and Orlik-Rückemann's group. Fighting went on into October.
1943
1944
- 4 January 1944EventThe Red Army crosses the pre-war Polish borderOn the night of 3–4 January 1944 troops of the 1st Ukrainian Front crossed the pre-war Polish border near Rokitno in Volhynia.
- 15 January – 26 October 1944BattleOperation TempestFrom January to October 1944 the Home Army struck the retreating Germans and greeted the Red Army as hosts. The Soviets disarmed its units.