Final answer:
Approximately 10% of the American population joined the war effort during World War II.
Step-by-step explanation:
During World War II, approximately 16 million American men and women served in the armed forces, which made up about 10% of the total population in the country. This number includes both those who volunteered and those who were drafted into service. It is important to note that not everyone in the population was eligible or fit to serve, which is why the percentage is not higher.