World War I (1914-1918).
This photo was taken by Paul Popper for Getty Images. It shows British troops running out of their trenches to charge the enemy lines during the bloody Battle of the Somme. Spaces between trenches were branded "no man's land."
When Americans got to Europe, this is what the fighting looked like. Life in the trenches involved sharing damp, diseased quarters with dead bodies and making slow, if any progress.
The Great War between the Allied and Central Powers involved more than 30 countries. It's estimated that 8.5 million soldiers and 13 million civilians were killed during the war.
World War II (1939-1945).
This photo was taken by Jose Rosenthal for the Associated Press. It shows US troops raising the flag on the Japanese island of Iwo Jima, one of the first pieces of Imperial Japanese territory to fall to US hands.
Fifty years after the picture was taken, the Associated Press wrote that it may be the world's most widely reproduced.
But half of the six soldiers depicted died — among 6,821 Americans — on the very same island they claimed: Franklin Sousley, Michael Strank, and Harlon Block.
The Second World War between the Allied and Axis Powers killed approximately 40 to 50 million people, making it the deadliest war in history.
Korean War (1950-1953).
This photo was taken by R.V. Spencer for Getty Images. It shows a Korean girl carrying her brother past a stalled M-26 tank in 1951.
The Korean War began in June 1950 when North Korea, backed by the Soviet Union and China, invaded South Korea, which was backed by the US.
The fighting lasted until 1953, and was devastating to the local population, with approximately 1.6 million civilian casualties on both sides. The war, however, is still technically ongoing, as there was never a peace treaty.
The Vietnam War
This photo was taken by Nick Ut for the Associated Press.
The image shows South Vietnamese children running after a South Vietnamese plane accidentally dropped napalm on its own soldiers and civilians during the Vietnam War in 1972.
The naked girl, Kim Phuc, had ripped off her burning clothes while fleeing.
The picture became symbolic of the atrocities of the Vietnam War, which killed about 58,200 US troops and as many as 2 million civilians.
The US officially entered the war in 1965 to stop the halt the spread of communism, eventually pulling out in 1973 with nothing to show for it.
The Gulf War (1990-1991)
This photo was taken by a member of the US Air Force.
It shows US fighter jets flying over Kuwaiti oil fires, which the Iraqi Army had lit as they retreated from the country after invading.
The US invaded Kuwait in 1990 to oust Iraqi forces, which had invaded Kuwait over alleged unfair oil drilling practices, among other things.
The US and its allies quickly chased away the Iraqi army, but the short war was largely seen as a war over oil. The US left Iraqi President Saddam Hussein in power.
The War in Iraq
This photo was taken by Getty Images photographer Chris Hondros in Iraq in 2005.
The image shows 5-year-old Samar Hassan after US troops had accidentally killed her parents at a checkpoint in the town of Tal Afar.
The photo ran in newspapers and media outlets around the world for days, forcing the US military to change how it operated checkpoints and further questioned the US' role in Iraq.
In 2003, the US invaded Iraq and overthrew Saddam Hussein's regime because the Bush administration said he had weapons of mass destruction, most notably nuclear weapons, which turned out to be untrue.
The US occupied the country until 2011.
War in Afghanistan (2001-Present).
This photo was taken by Reuters photographer Goran Tomasevic in Afghanistan in 2008.
The image shows US Marine Sergeant William Bee nearly getting shot after Taliban fighters opened fire near Garmser, Helmand Province.
US service member deaths have decreased since the US military stopped conducting patrols in Afghanistan, but before that, US troops were essentially used as bait to draw out Taliban fighters.
The US invaded Afghanistan in 2001 to defeat al-Qaeda and the Taliban, but after more than 17 years, there's been little progress stabilizing the war-torn country.
Read more about the photo here.