A dichotomy is a division into two parts, and according to Dictionary.com, divisions can cause tensions or conflict.
After World War I, the United States was a nation in conflict. With the decline of Great Britain as the premier world power, the United States, a wealthy, powerful country, was clearly poised to take on a leadership role in world affairs. The US had been on the winning side of a world war. It had also long had largely open borders which helped connect it to the rest of the world. It welcomed in immigrants freely from all over Europe to man its growing industrial base.
The war, however, created a backlash. Many Americans wanted to retreat back into the United States's nineteenth-century isolationism, a position recommended by George Washington in his Farewell Address as a way for the country to protect itself while it stabilized and grew, but one that was possibly no longer applicable to its new status. The US refused, for example, to join the League of Nations, despite it being a pet project of president Woodrow Wilson. It also established strict immigration quotas that severely restricted immigrants from countries considered undesirable.
Thus, while many wanted the nation to take on the mantle of world leadership, which it obviously could do, many others wanted to reject new ideas about the country as a military and diplomatic leader and pushed for the US to stay out of world affairs.