Q:

Evolution of United States' world role?

This is a pretty big and general question, but I'm interested to hear some opinions from fellow StackExchangians:
Over the 20th century, how did the United States evolve into being a world superpower?

Generally, I think the USA's progression can be grouped into 4 periods, with the "superpower" phase starting around 1945:

Discovery - emergence of a new country: Constitution, government structure, values of freedom and democracy, radical new vision
Development - domestic changes: Civil War, Industrial Revolution, immigration, cities, growing inward power
Looking Outward - international affairs: Cuba, Philippines, WWI, WWII
Superpower - starts with atomic bombs on Japan, fully involved with world affairs, Korean War, War in Vietnam

A:

If the question is limited to

Over the 20th century, how did the United States evolve into being a world superpower?

then the answer is that in 1939 the United States, while a great power in the sense that its military posture and diplomatic interest was essential to the great power system of international relations, that by 1945 the United States had recast the world system in the image of its antagonism with both the Soviet Union and the remaining European powers with significant colonial assets (Britain, France, the Netherlands).  In 1939 the United States existed as one agent of similar military and political clout as most other peak agents.  In 1945 the United States and the Soviet Union stood alone as peak imposers of political systems and peak possessors of military force.
The underlying changes within United States society that enabled this change in position were primarily economic, and relate to Fordism-Taylorism. (cf: Mandel, Braverman, etal).  The United States found part of the escape from the cycle of periodic depression and class warfare in the factory through the deskilling of labour, the imposition of scientific control of labour, and through higher rates of remuneration for white, male, skilled workers.  The United States then proceeded to export this Fordist model to other societies, or sought to impose it on other societies.  By comparison, the only other society (prior to 1945) to be seriously dominated by Fordist-Taylorist conceptions of economic management of the firm and labour was the Soviet Union.  Other capitalist economies were using outdated techniques, small production scales, and labour management practices that were comparatively less "efficient" in producing surplus value or gross output of use-values.
The underlying changes within the world system were: the defeat of Japanese reactionary capitalism by the KMT and CCP (and millions of Chinese people); the defeat of German fascist capitalism by the Soviet nomenklatura (and millions of Soviet people); and the neutering of European capitals (Italian, French, Dutch and British) through war expenses, capital wear, cash and carry, and outright destruction by primarily US capital.  (Yes, this is a simplification, but it draws attention to the structure of economic outcomes).  This left the only significant world powers, economically or militarily, as the United States and the Soviet Union.  Other powers: China; the sub-continent's nationalist movements; defeated Japan, Italy and Germany; the minor European states; and France and the United Kingdom lacked such military or economic power.
Moreover, the implementation of the Marshall Plan, combined with the United States' NATO enforced requirements on Europeans, combined with the United States' support of nationalists in colonial areas, further undermined the military, economic and political power of Western Europe.  In many cases the United States achieved significant market penetration, eventually causing "neo-colonial" status in terms of the dominance of US capital in these markets.  Apart from more productive capitalism at home, the United States benefited from its political-economic imperialistic expansion, in the Marxist sense. (Hilferding, Lenin, Wallerstein)
The United States became a superpower because:

The great-power system collapsed (in favour of super-power system)
The United States had a more productive economy
The United States' foreign policy that supported, as a matter of course:

the destruction of great powers' status, 
export US structure of labour management along with US capital ownership.

In both cases, decisive causes appear to arise between 1890 and 1920, when Fordism and Taylorism were developed within the United States.  This (incidentally!) was the period of crisis of the previous labour-control method and attempts to enforce equilibrium development on capitalism.

A:

There are three large reasons why the US became the world super power: last man standing, the bretton woods institutions, and being the home of democratic capitalism.
Last Man Standing
Most of the rest of the world was devastated by WWII. The US on the other hand suffered nowhere near as many casualties, and its economy was booming by the end of the war. Well, when there are a ton of countries that need rebuilding and only one country that has overcapacity, what do you think happens? The US was only ever invaded during WWII in the Aleutian Islands which did not impact the country all that much.
Bretton Woods Institutions
The Bretton Woods Institutions are the World Bank and the IMF. These were established in Bretton Woods, NH before the end of WWII and established the monetary system for the post-WWII order. Both institutions are located in the United States and heavily influenced by the US. If you control the money flow of the world then you have a large say in how that world is shaped.
Home of Democratic Capitalism
As the home of democratic capitalism, which at the conclusion of WWII was chosen as the preferred model of development for the world's countries, the US had a natural head start on dictating world affairs. Most of the ideas propagated by the Bretton Woods system had their roots here in the US. The Soviet Union, and to a lesser degree China, attempted to export their world view, but ultimately they did not succeed.
In sum, the US held all the cards at the end of WWII and it would have been rather difficult for the US to NOT become the world super power it eventually did become.

