There are two major causes: Germanic tribes moving into the Western roman Empire until its fall by Odacer in 476 A.D.. On the other hand, emperor Constantine I had chosen Byzantium, and ancient greek site, as the "New Rome", which would become Constantinople. Constantine I established Christianity as the official religion for the empire.The eastern half of the roman empire survived more than 1000 years, because of strategic location as a trading center and connection between Europe and Asia.
It is A and C.
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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.