Complex sentence: It is quite reasonable to assume that Army Group Center would have taken Moscow by late September or early October if it began its offensive in August, but thereafter would have temporarily stuck in the mud caused by the seasonal October rains (as occured).
Split into simpler sentences: It is quite reasonable to assume that Army Group Center would have taken Moscow by late September or early October if it began its offensive in August. Thereafter, it would have temporarily stuck in the mud caused by the seasonal October rains (as occured).