Bernard Montgomery often assigned blame to the failure of his operations, such as Operation Market Garden, to the Polish troops under his command. Poland's relationship with the USSR during World War II was complicated. The main western Allies, the United States and the United Kingdom, understood the importance of the Soviet Uion in defeating Germany, to the point of refusing to condemn Soviet propaganda which vilified their Polish ally. The western Allies were even willing to help cover up the Soviet massacre at Katyn.