Does anyone have any ideas how to help answer this question, even the littlest bit could help?

At the beginning of the 20th century the US was not considered a world power in spite of its economic and demographic potential. By the end of the century it had become the sole superpower. Describe the ascendancy of the US in the 20th century. What key factors, events, people and/or situations contributed to this transition? why? how might china be in a similar situation as we start the 21st century?