Americas

The Mexican-American War (1846–1848), explained: the United States annexed Texas in 1845, disputed whether its border was the Nueces River or the Rio Grande, and fought Mexico across the north, California and the road to Mexico City. The Treaty of Guadalupe Hidalgo (2 February 1848) ceded some 525,000 square miles to the United States — 55 percent of Mexico's pre-war territory on the U.S. Office of the Historian's reckoning — for $15 million. The standard U.S. tabulation counts 13,283 American dead, most from disease; historians estimate about 25,000 Mexican dead.

The Chaco War (1932–1935), explained: landlocked Bolivia and Paraguay fought over the Chaco Boreal, a waterless wilderness of about 100,000 square miles thought to hold oil. About 100,000 men died, Paraguay won three-quarters of the disputed land in the 1938 settlement, and historians still argue over whether oil caused the war at all — the best archival work finds no known deposits in the disputed Chaco and no hard evidence the oil companies started it.

The Paraguayan War (1864–1870), explained: Paraguay under Francisco Solano López fought the Triple Alliance of Argentina, Brazil and Uruguay in the bloodiest interstate war in Latin American history. It claimed some 200,000 lives, cost Paraguay about 40 percent of its territory, and left historians arguing ever since over whether Paraguay lost a fifth of its people — or two-thirds of them.

The Spanish-American War explained: from the sinking of the USS Maine in Havana harbour on 15 February 1898 to the Treaty of Paris on 10 December 1898, the United States destroyed Spain's fleets at Manila Bay and Santiago, took Puerto Rico, Guam and the Philippines, and set Cuba up as a protectorate. Only 385 Americans were killed in action — disease killed more than five times as many — while the Cuban war that caused it all had already killed at least 170,000 civilians in Spain's reconcentration camps.

The American Revolutionary War explained: from 19 April 1775 to 3 September 1783, the Thirteen Colonies fought Britain for independence, won French and Spanish help after Saratoga, forced Cornwallis to surrender at Yorktown, and gained recognition in the Treaty of Paris — at a cost historians still count in ranges, because disease killed far more soldiers than battle did.

The Falklands War explained: from 2 April to 14 June 1982, Argentina and the United Kingdom fought a 74-day undeclared war over the Falkland Islands (Islas Malvinas), South Georgia and the South Sandwich Islands. Argentina invaded; a British task force sailed 8,000 miles to retake them. 907 people died, Argentina's junta fell, and the sovereignty dispute remains unresolved.

The War of 1812 (1812–1815) began with British seizures of American ships and sailors and ended in a draw the treaty barely mentioned. Fought mainly over neutral trading rights, it failed to conquer Canada, burned Washington, killed some 37,000 people across three nations — and left the United States, Canada, and Britain's relationship permanently changed.

The American Civil War (1861–1865) killed between 620,000 and 850,000 soldiers, ended slavery, and redefined the United States. Here is what caused the war, what it cost, and how it reshaped America.