7.Across the Potomac River is the Pentagon,a strange, low,five-sided building.This is the headquarters of the American army,and is also the American Ministry of Defense.During World War II more than 35,000 people were employed here,which gives you an idea of its size. 用7. Across the Potomac River is the Pentagon, a strange, low, five-sided building. This is the headquarters of the American army, and is also the American Ministry of Defense. During World War II more than 35,000 people were employed here, which gives you an idea of its size