Germany didn't become a country until 1871, when 39 sovereign kingdoms and states were unified. In 1919, after the end of World War I, the Germans were forced to sign the Treaty of Versailles. Among the demands of the Treaty, Germany had to
- Accept full responsibility for causing the war.
- Accept that the German emperor would be tried as a war criminal.
- Substantially reduce the size of their military.
- Give up control of German colonies and relinquish vast quantities of land to surrounding countries.
- Pay unbelievable amounts of money to the nations with which Germany was at war (the Entente Powers). Some of this debt would not be paid in full until the year 2020.
Let's be honest: Germany was largely responsible for the start of World War I, but while the Entente Powers (including Russia, Great Britain, France, Japan, and the United States) saw the Treaty of Versailles as fair punishment, the German people were humiliated and grew very resentful of the allies. Add to this the Stock Market Crash of 1929 — which caused the U.S. government to call in its loans to Germany — and soon, Germany was in the throes of a worse Depression than the United States: German unemployment reached 15%; people were homeless and starving in the streets; and what little money the government did have was being handed over to other nations as war reparations.
Germany was still a young country (barely 50 years old), and its people didn't have a sense of German "identity" yet. Adolf Hitler was a brilliant speaker and a strong organizer. He used all this German disenfranchisement to his advantage. He motivated the people, developed a sense of German pride, and convinced them that they deserved better. Eventually, Hitler convinced the Germans to go to war again, and World War II was underway. In a sense, Hitler brainwashed a nation.
The unprovoked invasion of neighboring countries and the murder of over six million innocent people aside, Hitler's war effort is credited with causing several scientific advances and the creation of products we still use today (the Volkswagen, rocket engines, and jet propulsion are three examples). Hitler also built a much needed infrastructure in Germany, including dams, railroads, and the world's first national highway system (the Autobahn). I suppose you could say these are positive contributions to the world.
But I think Adolf Hitler's most important contribution to the world is this: He showed us what unchecked greed, unabashed bigotry, and loathing and hatred for humankind can do. Hitler has given us an example of the worst kind of person, and the worst thing that can happen to humanity.