10. Mexican-American War
Fought between 1846 and the start of 1848, the Mexican War cost the United States $2.4 billion, the 10th-most expensive war in U.S. history. Texas, having gained its own independence from Mexico a decade earlier, had still not been granted statehood by the U.S. Texas’ annexation would have upset the equilibrium between slave and free states established by the Missouri Compromise of 1820. Mexico hoped to capitalize on U.S. indecision when its army crossed the Rio Grande to attack U.S. forces in the battle of Palo Alto. The war ended on February 2, 1848, with the Treaty of Guadalupe-Hidalgo, which established the Rio Grande as the southern border of Texas and settled U.S. acquisition of land in present-day California, Utah, Nevada, Arizona, and New Mexico for $15 million.
9. American Revolution
The nearly nine-year quest for independence cost the American colonies just over $2.4 billion, and nearly 4,500 lives in battle. While the war began as a revolt against unjust taxation, it ended with the Founding Fathers rejecting the social and political structures of Europe in favor of a democratic republic. The Treaty of Paris ended the war in 1783, officially recognizing the U.S. as an independent country and establishing its borders.
8. Spanish-American War
Often dubbed the first “media war,” sensationalized journalism helped fuel America’s support for involvement in the Cuban quest for independence from Spain. When the U.S.S. Maine, which had been sent to Havana to protect American interests, unexpectedly blew up in 1898, cries for American intervention increased. Congress officially declared war under the Monroe Doctrine — a foreign policy agenda that proclaimed a U.S. right to intervene in regional conflicts in the Western Hemisphere — and crushed Spanish forces worldwide in less than nine months. The war cost $9 billion and the U.S. acquired Guam, Puerto Rico, and the Philippines.