How the Allies might break Fortress Europe without the Soviet war

If the Soviet Union remained neutral and Germany never invaded it, how could the Allies break Fortress Europe?

Atomic bombing is the proposed outcome, by analogy with Japan; a conventional campaign plan is not developed.

The scenario removes the Soviet-German war and asks how the Western Allies could break Fortress Europe. Atomic bombing is the proposed eventual answer. The comparison is with the use of atomic weapons against Japan. No alternative invasion plan or timetable is developed in this brief counterfactual.