The Vistula–Oder offensive (Russian: Висло-Одерская операция, romanized: Vislo-Oderskaya operatsiya) was a Red Army operation on the Eastern Front in the European theatre of World War II in January 1945. The army made a major advance into German-held territory, capturing Kraków, Warsaw and Poznań. The Red Army had built up their strength around a number of key bridgeheads, with two fronts commanded by Marshal Georgy Zhukov and Marshal Ivan Konev. Against them, the German Army Group A, led by Colonel-General Josef Harpe (soon replaced by Colonel-General Ferdinand Schörner), was outnumbered five to one. Within days, German commandants evacuated the concentration camps, sending the prisoners on their death marches to the west, where ethnic Germans also started fleeing. In a little over two weeks, the Red Army had advanced 300 miles (483 km) from the Vistula to the Oder, only 43 miles (69 km) from Berlin, which was undefended. However, Zhukov called a halt, owing to continued German resistance on his northern flank (Pomerania), and the advance on Berlin had to be delayed until April.