On the southern frontier of East Prussia, the regulars of the locally recruited German XX Corps, reinforced by large contingents of Landwehr and fortress troops, prepared to meet the Russian Second Army. While the corps had the mission of delaying the Russian advance, the troops and junior officers were not interested in yielding any Prussian ground.

Though the Germans deployed for a conventional defense, when the Russians appeared all planning and doctrine instantly disappeared. Like "murderous schoolboys," historian Dennis Showalter wrote, German battalions went forward with flags flying and bands playing, some of them supposedly bellowing the Deutsclandlied ("Deutschland Uber Alles"). Russian fire cut down many before small but fierce close-quarters fights broke out all along the line. In the village of Lahna, 500 Prussian Jägers engaged the Polish regulars of the Russian 8th Division all day in no-quarter house-to-house fighting; perhaps 30 men limped away when darkness fell.