On 25 April 1898 the United States Congress

United States Congress

The United States Congress is the bicameral legislature of the federal government of the United States, consisting of the Senate and the House of Representatives. The Congress meets in the United States Capitol in Washington, D.C....

 declared war upon Spain, in response to that country's declaration of war the prior day. The Spanish–American War ensued, resulting in a decisive victory for the United States, and ending with the Treaty of Paris

Treaty of Paris (1898)

The Treaty of Paris of 1898 was signed on December 10, 1898, at the end of the Spanish-American War, and came into effect on April 11, 1899, when the ratifications were exchanged....

on December 10 of the same year.

Text of the declaration

Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, First. That war be, and the same is hereby, declared to exist, and that war has existed since the 21st day of April, A.D. 1898, including said day, between the United States of America and the Kingdom of Spain.

Second. That the President of the United States be, and he hereby is, directed and empowered to use the entire land and naval forces of the United States and to call into actual service of the United States the militia of the several States to such extent as may be necessary to carry this act into effect.

Approved, April 25, 1898.