How did World War 2 affect the power of Western imperialist nations?

Answer 1

Germany, Japan and Italy lost- and became governed by the countries that won. Much of Europe and Asia was severely damaged by the war. The US ended the war with little damage to its cities, agriculture and manufacturing, and was the sole possessor of atomic weapons- shifting power heavily to the US.

Answer 2

This should be a long and complicated answer, as the impact (long- and short term) for each major power has been the subject of numerous books. A very brief summary could be as follows:

- Germany lost its sovereignty for a time and stayed divided until 1989. It lost its status of powerful military nation, removed the military brass from all positions of strategic influence and stayed out of any military involvement for more than 50 years. Thanks to US-sponsored restoration plans after the war, it managed to quickly restore its annihilated infrastructure and industrial base and over time became the most powerful economic force in Europe.

- Britain lost its leading role and power as "British Empire", being crippled financially by the war and losing most of its colonies, India first of all, soon after the war: many colonies - especially in the Far East - had seen Britain brought to its knees by Japan, and that had inspired many local independence movements. Moreover, the US refused to support Britain or any other country to hang on to its colonies.

- the US - until then always isolationist - took over Britain's role as premier worldwide military and economic power. Because of concerns about Communism, it started to throw its weight about and entered into alliances all over the globe. Its economy and industry got an enormous boost because of catering for most of the Allies' war requirements and it maintained its prominent position and economic growth for decades afterwards.

- Japan got the same kind of US help that Germany got to rebuild its economy after the war. Like Germany, it totally 'demilitarized' its economy and political structure and like Germany, it proved very adept at quickly rebuilding its destroyed infrastructure and industry. For many decades afterwards, it was the most powerful economy in the region.

- Russia (then: the USSR) made a lot of hay in political terms from winning the war in Eastern Europe: it became and remained until 1989 the dominant power in all of Eastern Europe, installing Communist governments in all countries, including its part of occupied Germany which it established as a separate, Communist country. Getting no US Marshall Help for economic recovery, it had to restore its massive war damage by itself, and did this partly by helping itself from the countries it now controlled. Winning the "Great Patriotic War" established Stalin's personal prestige and the position of the Communist party in Russia to an absolute level until the late 80's.