German forces were outnumbered twenty to one in aircraft, eleven to one in infantry, and seven to one in tanks on the Eastern Front.[247] By the end of the month, the Red Army had made bridgeheads across the Oder, the last geographic obstacle before Berlin.[248] The western Allies continued to advance as well, but not as rapidly as the Red Army.[249] The Panzer Corps conducted a successful defensive operation on 17–24 February at the Hron River, stalling the Allied advance towards Vienna.[250] The 1st and 2nd SS Panzer Corps made their way towards Austria, but were slowed by damaged railways.[251]