Final answer:
The Alien Enemy Act authorized the president to detain and expel citizens of a country with whom the United States is at war, such as during World War II.
Step-by-step explanation:
The act that authorized the president to detain and expel citizens of a country with whom the United States is at war is the Alien Enemy Act. This act was used during times of war to target foreign nationals from enemy nations who were suspected of disloyalty and posed a potential threat to national security. One example of the Alien Enemy Act being implemented was during World War II, when President Roosevelt signed Executive Order 9066, authorizing the removal of Japanese-Americans from exclusion zones and their internment in government camps.