Complex sentence: By the time it attacked, the German troops' resistance had stiffened, and the corps became bogged down in heavy fighting in the Tyagaevo area, under air attacks which destroyed many of its Lend-Lease tanks.
Split into simpler sentences: By the time it attacked, the German troops' resistance had stiffened. It engaged in heavy fighting in the Tyagaevo area, subject to air attacks which destroyed many of its Lend-Lease tanks.