Em. After Molotov-Ribbentrop, the Soviet Union tried to formally join the Axis as the fourth Axis power.

It's hard to argue that was to buy time, especially given they had spent more effort conquering their neighbors and helping the Nazis than building defenses against the them. They just wanted a larger chunk of Europe and Western Asia.

Their attempt failed because Stalin got greedy with what chunk of Europe he wanted and their poor performance against Finland convinced the Nazis to double cross them and invade.