Later, Sulik became commandant of the 5th Border Infantry Division, part of General's Władysław Anders Polish II Corps.
Among those, who gave their lives fighting the German aggressors, was the Regiment's Captain Władysław Raginis Those units that remained in Wołyń Voivodeship, manned the Sarny Fortified Area, which guarded the Polish-Soviet border.
On September 17, following the Soviet aggression on Poland (see: Molotov–Ribbentrop Pact), the Regiment's forces engaged in heavy fighting with more numerous and much stronger troops of the Red Army's 60th Rifle Division.
Polish soldiers fought to the last bullet, halting the Soviet advance until September 20.
The Soviets, unable to break the defenses, set bunkers on fire, and also murdered a number of Polish soldiers.