The real problem was that tooling up and delivering trained men and war material needed every month they had before D Day and D Day would not have happened had the USSR been finally defeated.
They really went when it was no longer possible for Germany to divert a corp or two from the East. to properly stop the allied buildup and onslaught. Even then they had their moments.
In the interim it made excellent sense to shift emphasis to North Africa where Germany was strategically limited and the allies could use already raised and trained forces. The bulk of those forces ultimately held down German forces in Italy and later provide a springboard for the invasion of Southern France.
Everyone knew the alliance with the USSR was one of convenience. it was actively betrayed the moment the fighting stopped.