7. American Civil War
The Civil War claimed more American lives — 750,000 — than all other U.S. conflicts combined, and cost both sides nearly $80 billion. Fought primarily over the issues of slavery and states’ rights, such as taxation and representation, the Civil War began after South Carolina seceded from the Union and fired upon a Union merchant ship heading to Fort Sumter with supplies. The standoff in Charleston Harbor continued until April 1861, when the war officially began. Four years later, General Robert E. Lee of the Confederate States of America surrendered to Ulysses S. Grant at Appomattox Court House, Virginia.
6. Gulf War
The Persian Gulf War was one of the shortest conflicts in U.S. history, costing $102 billion, or just 0.3% of GDP in 1991. Tensions mounted when Iraqi dictator Saddam Hussein invaded Kuwait. Despite strong calls from the United Nations for Iraq to withdraw, Hussein refused. A few months later, a U.S.-led coalition initiated Operation Desert Storm. The offensive lasted 42 days. As the first major conflict after the Cold War, the Persian Gulf War was, at the time, heralded as a success for the international coalition.
5. World War I
War broke out in Europe in 1914 but the U.S. remained neutral for the next three years. However, after Germany reneged on its pledge to respect the neutrality of U.S. ships in the Atlantic, and tried to entice Mexico into declaring war on the U.S., President Woodrow Wilson asked Congress for a declaration of war on April 2, 1917. The war ended 19 months later with the Treaty of Versailles. Ultimately, the war cost the U.S. $334 billion, or nearly 14% of GDP in 1919.