Final answer:
The United States shifted from isolationism to active engagement in world affairs during the 20th century due to its growing economic and military power, the realization that traditional European powers could no longer maintain global stability, and the advent of major conflicts like World War I and World War II.
Step-by-step explanation:
The United States began taking a more active role in world affairs in the 20th century due to the need to exert its influence following its emergence as an economic power. The country's expansionist policies, known as imperialism, initially focused on regions outside of Europe, carving out territories of control in Africa, Asia, and the Pacific. With the arrival of the 20th century, significant events such as World War I and later World War II catalyzed the U.S. transition from isolationism to a proactive foreign policy.
After World War II, the United States stepped up to fill the void left by weakened traditional powers like Great Britain and France. The establishment of the Monroe Doctrine and advice from past leaders like George Washington initially kept the U.S. from entangling alliances with European nations. Nonetheless, the U.S. found itself involved in Great Power conflicts, leading to expanded international diplomacy and military presence.
As a consequence, the United States moved towards a more interventionist stance in the global arena, participating actively in international diplomacy, economic affairs, and conflict resolutions to maintain stability, support allies, and advance its strategic interests. This included key roles in forming institutions such as the United Nations and NATO, and engaging in the broader geopolitical struggle of the Cold War.