1. Korean War
   Civil War
2. WWI

The American Civil War claimed more than 600,000 lives. Bullets and diseases claimed lives by the scores, making it the deadliest war in the nation’s history.

1. Civil War
   Vietnam War
2. WWII

For more than 17 years, American troops served in the jungles of the Vietnam War. And when they returned home, many of them faced scorn from their fellow citizens.

1. Moro Rebellion
   Barbary War
2. WWII

America didn’t want to be involved in WWII, but after Japan smashed Pearl Harbor, the gloves came off. The U.S. went into survival mode and churned out soldiers and guns by the millions.

1. WWI
   Mexican-American War
2. Northwest Indian War

Starting in 1785, the U.S. went to war against an alliance of Native Americans in the Northwest Indian War. American troops were handily defeated often in the early parts of this war.

1. Civil War
   Mexican-American War
2. Vietnam War

When Abraham Lincoln was elected president, the South knew many of its traditions — including slavery — were at risk. The Confederacy seceded from the Union and the Civil War was on.

1. WWII
   Korean War
2. American Revolution

In April 1775, British troops tried to take weapons from colonists in the New World. The colonists resisted in the Battle of Lexington and Concord, and the American Revolution began in earnest.

1. WWI
   WWII
2. Spanish-American War

In late 1944, Hitler mounted a final desperate offensive that became known as the Battle of the Bulge. His men trapped parts of America’s 101st Airborne, who spent days fighting for their lives before reinforcements arrived.

1. Iraq War
   Korean War
2. WWI

In 1914, a Serbian extremist murdered an Austrian archduke and started political chaos. Eventually, it became WWI ... a fight that would ultimately involve American fighters.

1. WWI
   Korean War
2. Vietnam War

In 1950, North Korea invaded South Korea, and the U.N., led by the U.S. moved to repel the attackers. The Korean War wound up dragging on for years.

1. American Revolution
   Moro Rebellion
2. Whiskey Rebellion

Starting in 1899, the Moro people resisted U.S. troops in control of the Philippines, which American claimed after the Spanish-American War. In 1913, after about 14 years of resistance, the Moro Rebellion was finally won by the U.S.

1. WWI
   Civil War
2. Vietnam War

President Wilson avoided taking American into WWI until 1917, and by then, Europe was in tatters. American troops fought and died for about a year and a half during the Great War.

1. Civil War
   Mexican-American War
2. Spanish-American War

In the 1840s, Mexico and America clashed over lands in the Southwest. In the end, American troops invaded and captured Mexico City.

1. Civil War
   WWII
2. Vietnam War

In the Civil War, engineers and weapons makers found new ways — like machine guns — to expedite the slaughter. Both the North and South used primitive machine guns in this conflict.

1. WWI
   War of 1812
2. Civil War

During the War of 1812, American and British troops took turns abusing their power. During a brief occupation of Washington D.C., the British burned the White House into ashes.

1. Iraq War
   WWI
2. Korean War

Matthew Ridgway was a WWII hero who was called to help the Korean War effort. His masterful intervention altered U.N. strategies and helped the U.S. and South Korea beat back the North Koreans and Chinese.

1. Korean War
   WWI
2. WWII

Patton was part general, part media hound ... but he was all business on the battlefield. Time and again, he sent Axis troops reeling in the Second World War, and German generals feared his aggressiveness.

1. Mexican-American War
   Civil War
2. WWII

In August 1945, President Truman felt that atomic weapons might be the only way to stop the Japanese. He ordered two atomic bombs dropped on Japan. Days later, the war finally ended for good.

1. WWI
   Texas Revolution
2. Spanish-American War

In 1835, Texas settlers took up arms against the Mexican government, which sent an army to put down resistance. At a fortress called the Alamo, Mexican troops killed nearly everyone — and made martyrs of the Americans they killed.

1. WWI
   Civil War
2. Korean War

Neither side officially declared war during the Korean War, which became known as a "police action." For anyone who served there, it was a bloody and terrifying war, plain and simple.

1. Civil War
   Persian Gulf War
2. Korean War

When Iraq invaded Kuwait in 1990, President Bush made good on his threats to attack Iraq. The Persian Gulf War was a brief but spectacular made-for-TV special starring Saddam Hussein as the mustachioed villain.

1. WWII
   Spanish-American War
2. Mexican-American War

On June 6, 1944 — D-Day — Americans were cut down in the greatest numbers in U.S. history. At least 2,500 men died that day storming the shores of Normandy.

1. Northwest Indian War
   Civil War
2. Korean War

In 1791, 1,000 troops led by General Arthur St. Clair were routed by warriors in the Northwest Indian War. The defeat angered President Washington, who was then inspired to beef up his efforts to beat the Indians ... and he did exactly that.

1. Red Cloud’s War
   WWI
2. Vietnam War

In WWI, American troops massed for the biggest offensive in U.S. military history, the Meuse-Argonne Offensive. The attack lasted for just over a month but cost more than 26,000 American lives.

1. Korean War
   Mexican-American War
2. Vietnam War

President Polk attempted to purchase disputed lands in the Southwest. But Mexico, divided by its own messy political situation, didn’t even consider the offer.

1. Civil War
   Korean War
2. WWII

In 1862, the North and South clashed in the Battle of Antietam. Altogether, more than 3,600 men died — and there were nearly 23,000 total casualties, all in the span of a single day.

1. Korean War
   WWII
2. Civil War

In 1972, Alan Alda started his run as Hawkeye Pierce in "M.A.S.H.," which became one of TV’s most-watched shows. The program often highlighted the seriously depressing side of the Korean War.

1. Spanish-American War
   Persian Gulf War
2. WWI

In 1898, the USS Maine exploded (for reasons unknown) near Cuba, causing America to became entangled in Cuba’s uprising against Spain. Then the chaos snowballed into the Spanish-American War.

1. WWII
   War in Afghanistan
2. Iran War

Since the 9/11 attacks, the U.S. has been embroiled in the War in Afghanistan, which is now approaching its 17th year. Unless the U.S. withdraws soon, it will become the longest war in U.S. history.

1. Iraq War
   WWII
2. Civil War

In the Civil War, engineers came upon the idea of plating their ships with iron. These "ironclads" were much more resilient in combat and sparked a revolution in warship engineering.

1. Spanish-American War
   Vietnam War
2. WWI

In 1964, a reported clash between North Vietnamese and U.S. ships became known as the Gulf of Tonkin Incident. The event — which may have been falsified by U.S. leaders — gave America cause to escalate the Vietnam War.