The United States become a global power following the conclusion of WWII. It helped create and oversea international institutions and alliances, such as the United Nations and NATO. It took a more active role in the affairs of countries around the world, and maintained a huge military, rather than downsizing after the war ended.
These actions helped to ensure peace in Europe, but also led to many interventionist wars around the world, such as Vietnam, Iraq, and Korea. It has also led to the United States being in serious debt, largely due to our intense military obligations.
The answer is A.
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