Months of continuous operations had taken a heavy toll on the Soviet forces and some divisions were reduced to 1,000–2,000 combat effective soldiers. On 19 February, Field Marshal Erich von Manstein launched his Kharkov counterstrike, using the fresh II SS Panzer Corps and two panzer armies. Manstein benefited greatly from the massive air support of Field Marshal Wolfram von Richthofen's Luftflotte 4, whose 1,214 aircraft flew over 1,000 sorties per day from 20 February to 15 March to support the German Army, a level of airpower equal to that during the Case Blue strategic offensive a year earlier.[6]