Why did Germany lose World War 2?
In point form:
1. Fighting a war on two (or three) fronts.
2. Fighting silly odds. Deciding to attack Russia was a bad idea, as well as declaring war on the US. Both of these countries could substantially outproduce Germany. Although Germany took the initiative both times, they may have had little choice. The United States would have entered the war one way or another due to the fact that Britian was their closest ally and they could not sit and watch them be destroyed by Germany.
3. Leadership. Adolf Hitler assumed personal operational command of the German armed forces and did not know how to develop effective tactics and strategies. Examples abound: attacking Stalingrad instead of Moscow, failing to follow through with the invasion of Britain, the development of the Me 262 as a bomber, setting Panzer and infantry rally points hundreds of miles behind enemy lines, failing to divert sufficient troops to Rommel in North Africa, and failing to mop up the BEF at Dunkirk.
4. Poor choice of allies. The alliance with Italy was a millstone round the Nazis' neck. Japan would only have made sense as an ally if it had attacked the Soviet Union in the Far East. Hitler's main alliance was based on ideology, not on shared interests. The other less powerful Axis states like Hungary and Romania tended to follow in step with Germany in an effort to gain territory.
The key year to look at is 1941. At the start of the year Germany had the initiative; but by the end of the year Hitler had thrown away that avantage. Early in the year Britain and the Commonwealth were of course still fighting, but their chances of invading and defeating Germany were nil. During 1941 Germany attacked the Soviet Union and declared war on the U.S. From December 1941 on Germany found itself reacting to events - which was a huge disadvantage. 1942 became the 'make or break' year for Germany, and after Stalingrad, Germany was only able to prolong the war - not win it or even negotiate a tolerable peace treaty.