Complex sentence: Soviet partisans had been disrupting the rear of the Army Group Center (and the German 4th Army in particular), and, in January 1942, they were strengthened by Soviet Airborne Troops (the Vyazma airborne desant operation, part of the Battles of Rzhev).
Split into simpler sentences: Soviet partisans had been disrupting the rear of Army Group Center (and the German 4th Army in particular). In January 1942, they were strengthened by Soviet Airborne Troops (the Vyazma airborne desant operation, part of the Battles of Rzhev).