which statement best describe the United States' changing role in the world during the twentieth century

Since 1917, when United States became involved in WW1, they have become the leading element in world politics. Their role in WW1 and WW2 was the most decisive one, and after WW2 United States were the world leading superpower. They are the member of every important world trading, politics and military association. In most of the important political turbulence they had leading role.

Explanation:

After abandoning Monroe's doctrine, United States were involved in number of conflicts around the globe. Through time their economy grew stronger and because of that can be seen as a leading force throughout the 20th Century in the area of economy, politics, technology, etc.