Calling the relationship between those countries an alliance is a big stretch. For all its leniency towards Germany during the interwar period, you'd have to call the US an ally to Germany as well, nevermind its infamously conciliatory treatment of the Nazis after the war (ie Operation Paperclip and the relatively rapid abandonment of denazification in West Germany). Furthermore, the US did not begin actually fighting the Nazis until after the USSR, and not until they were compelled by the Japanese attack on Pearl Harbor.

Regardless, we know what the Nazis did while they were in power and, as I previously cited, we know what they were going to do if they had prevailed. The fact is that after about four decades of Soviet rule, the Poles and other Eastern European peoples still exist and have thrived in comparison to the ethnic cleansing and extermination they would have faced under a victorious Nazi regime. Anyone whose family actually had an unfairly difficult time under the USSR has my condolences, but at least you're still around to express your resentment!