In which war did the United States defend its Monroe Doctrine after the sinking of the USS Maine?

The correct choice is the Spanish-American War because this conflict specifically arose from the United States' response to the sinking of the USS Maine in Havana Harbor in 1898. The explosion of the USS Maine, which was a U.S. naval ship stationed in Cuba, triggered public outcry and heightened tensions between the United States and Spain.

The Spanish-American War was significant for reaffirming the Monroe Doctrine, a principle established in the early 19th century that asserted U.S. opposition to European colonialism in the Americas. The war resulted in the U.S. defeating Spain and gaining territories such as Puerto Rico, Guam, and the Philippines. This shift in U.S. foreign policy marked an extension of American influence in the Caribbean and the Pacific, aligning with the Monroe Doctrine's aim to limit European intervention in the region.

The other options refer to conflicts (the War of 1812, World War I, and World War II) that do not directly relate to the defense of the Monroe Doctrine or the events following the sinking of the USS Maine. These wars have different causes and implications that do not reflect the specific circumstances surrounding the Spanish-American War.