The United States has declared war 11 times:

War of 1812 against the United Kingdom

Mexican-American war against Mexico

Spanish-American war against Spain

World War I against Germany

World War I against Hungary

World War II against Japan

World War II against Germany

World War II against Italy

World War II against Bulgaria

World War II against Hungary

World War II against Romania

It has not been at war since World War II. How wonderful! What a peaceful country!