Why did the United States declare war with Spain on April 21, 1898?

to test the strength of its newly developed military and navy

to annex the Philippines, Puerto Rico, and Guam

to avenge the sinking of a US battleship by "Butcher" Weyler

to free the Cubans and protect American interests

The United States declared war with Spain on April 21, 1898 to free the Cubans and protect American interests.

They wouldn't start a war just to test its army, so A is incorrect. B is also incorrect, because they didn't plan to annex these countries. C is incorrect because the ship which sank was Maine, and not Butcher.