What is the summary of how the Allies defeated Germany?

The Allies defeated Germany by the US and Britain sitting back and letting the Soviet Union do all the really bloody work.  For much of the war, the US and Britain were only really fighting German in the air and on the sea.  During that time, the Russians were losing huge numbers of men fighting Germany on the ground and weakening Germany considerably.

The Western Allies put some pressure on Germany by invading Northern Africa and then Italy, but the main ground fighting was still on the Eastern Front.  When D-Day came, Germany was finally forced to fight a two front war and quickly lost.

So the strategy was for the US and Britain to attack Germany's industrial capacity while the US built up the Allied arsenal.  As they did this, the Soviets did most of the fighting and dying.  When the arsenal was built up enough, the Western Allies invaded Europe and dealt the death blow to Germany.