On April 25, 1898, the US Congress officially declared war on Spain. The declaration stems from the sinking of the USS Maine as it was arriving in the Havana Harbor in Cuba, a Spanish colony, in January 1898 while insurgency was becoming a problem . The battle ship went to Cuba in order to protect American interests in the country as well as a show of force to Spain. But a mysterious explosion sunk the US Navy vessel in February 15 where a total of 266 out of 355 sailors in the harbor and killed.
This provocation, whether intended or not, caused many Americans to clamor for war. Due to pressure from the people, then President McKinley was forced to declare war on Spain through the US Congress. By April 25, 1898 Congress signed the declaration and the war was on. This war established the US for the first time as the new superpower. Its victory after a three-month war further confirmed this while the loss questioned the credibility of Spain as an empire.
It was this war that led to Spain giving the US the Philippines, Puerto Rico and Guam.