Although the United States gained its prominent role in the world since way before the First World War, the truth is that this major European conflict, along with the Second World War and the Cold War (with all of its included conflicts) established America as the leader in the international arena. Policies and laws were established before and since the two major wars that led the United States to act the way they did and changed the way that world politics, economics and power, were managed.
The first major involvement of the United States in the world arena that led to major changes in power control came during the First World War. Many of the policies that were taken at this time not only fostered the War but prevented a major player from entering it. This is the case of President´s like Woodrow Wilson, who in the beginning believed that American involvement in a European conflict, should not go beyond diplomacy. However, this changed when American interests, and world balance, were threatened majorly. The change in the American position, and their entering the war, led to a major shift that resulted in the resolution of it.
The second major involvement, in general terms, came with the Second World War, a conflict that truly reflected the power and importance of the United States in world balance. Before it, and during the rise of Nazi power in Europe, the United States maintained several policies that prevented it from further interfering in Europe. Standpoints like Isolationism, in the 1930s, policies that established the power of the United States in the Americas by prohibiting European powers from returning, and interfering, in Latin America, among others, showed the way the balance was leaning. Then, when World War II burst, America stood on the sidelines and it took an attack on their interests in Pearl Harbor, for the United States to take a standpoint and act. Again, the entrance of this nation tipped the balance of the war in favor of the Allied powers and ultimately, with other factors as well, helped to resolve it.
Finally, came the Cold War, this one, a direct conflict for power between two nations and their systems of government: the United States and the Soviet Union. Two major players in the arena of the Second World War, their confrontation led to major crises that almost disembarked the world into a third war. At this time, American foreign policy did shape the outcomes of many. For example, with the Nixon Doctrine, the United States once again refrained from militarily intervening in foreign conflicts and decided to shift to only diplomacy. There was also, during the 1970´s and beyond, major involvement of an indirect nature, on the situations being faced in the Middle East, mostly through acts from the CIA. With the arrival of the Reagan era, and a climax in the confrontation with the Soviets, America again began to establish policies of rearmament and reinforcement of military power to debilitate and destroy Communist influence in the world. These policies, and this race against Communism, led the United States to decide to involve themselves in two major conflicts: the Korean War and the Vietnam War, two fights that were very far from American soil and interests, but that became the central bastion of battle against the Communist power.