Advancing from East Prussia westwards into Polish territory, the German XXI Corps had a first-day objective of securing the city of Grudziaz (Graudenz) and pushing across the Osa river. The Polish 16th Infantry Division had only mobilized eight days before the invasion but Grudziaz was its home station and its soldiers quickly moved to the south bank of the Osa to defend their homes.

The Poles repelled repeated German assaults, until the Germans held back their infantry and allowed their artillery to bombard the Poles. Afterwards the invaders managed to get substantial forces across the Osa and maintain a bridgehead. Now the Poles attacked, with insistent fury, but despite their determination they had not thrown the Germans back across the river when night fell. Reinforcements arrived under cover of darkness, but a night attack coordinating units from two divisions was beyond Polish capabilities and the effort would have to await daylight.