Japan. America occupied Japan after WWII and Japan adopted what Douglas MacArthur later called the most liberal constitution ever. America turned Japan from a fascist-military dictatorship to a pacific, liberal democracy.

America also laid the foundations for Japan's economic miracle by helping Japan reconstruct its economy because it feared Japan would go communist otherwise. The Japanese economy boomed because it manufactured what the American armed forces needed to fight the Korean War.

Source: William Horsley and Roger Buckley, Nippon, the New Superpower: Japan since 1945 (1990).