General Marshall and General MacArthur - 1945 ...

In contrast to the Treaty of Versailles after World War I, the Allies had learnt from history. So, instead of punishing Germany and Japan to such an extent that they would re-arm and reassert themselves militarily, General George Marshall in Europe with his Marshall Plan and General Douglas MacArthur separately and, apparently, independently set themselves the task of rebuilding Germany and Japan.
             
                   

Some aspects of the Marshall Plan are ironic, such as that Germany was relieved of any debt, but fellow Allies like Britain had to pay back the US for war loans such as to supply aircraft and warships, which lasted till the end of 2006, 50 years later.

See:

• http://www.marshallfoundation.org/TheMarshallPlan.htm
• http://www.historynet.com/american-proconsul-how-douglas-macarthur-shaped-postwar-japan.htm
• http://news.bbc.co.uk/1/hi/6215847.stm (Britain pays back war loans to US)

Their two schemes were so successful that within a couple of generations, Germany (or at least West Germany as it then was) and Japan rose to become two of the world's largest economies rivalling and, in some cases beating, the economies of the victors.

The purpose was to enable Germany and Japan to become self-sufficient and not be a source of resentment and revenge. It was never to create formidable rivals!

Treaty of Versailles, 1918

After World War I, the victors through the Treaty of Versailles - http://www.historylearningsite.co.uk/treaty_of_versailles.htm -  imposed such harsh penalties, including punitive war reparations on Germany that the rise of Hitler (or someone like him) became inevitable.

This is one of the most clear cases of the Law of Unintended Consequences as the intention was to make it difficult if not impossible for Germany to threaten the rest of Europe!