The first war ever recorded took place in Mesopotamia in 2700 BCE, according to The Ancient History Encyclopedia.

The Greeks and Romans had customary laws of war, which the Western military tradition took inspiration from, such as war being openly declared and with legitimate cause, pledged word being kept, and a truce being given to the defeated so that they could recover and honor their dead.

While the idea of drafting able-bodied men may date back to Egypt in the 27th century BCE, the nationwide system most closely resembling that of today was introduced in France during the French Revolution, and institutionalized by Napoleon after he became emperor in 1803.

Conscription mandates the compulsory enlistment of citizens into the military, so countries with large populations have stronger military capabilities, and serving can help advance equality, per The Conversation.

As Russia's war with Ukraine has developed, countries in Europe like Latvia, Sweden, and Lithuania have reintroduced or expanded conscription mandates.

In 1917, the US Congress passed the Selective Service Act, which called all men ages 21 to 30 in the 48 states plus Washington, DC. In 1918, this expanded to include men ages 18 to 45 from Alaska, Hawaii, and Puerto Rico.

The Army, Navy, and Marines were founded in 1775, and the Coast Guard was founded in 1790. The Air Force wasn't formed until 1947.

During World War I, some pilots had to be trained with makeshift cockpits and pretend guns, per Getty.

The US tried to stay out of WWI, but joined after Germany re-launched submarine attacks on passenger and merchant ships in 1917, after having pledged not to, and tried to get Mexico to turn against the US, according to the Office of the Historian at the State Department.

There were an estimated 37 million deaths reported during World War I.

In 1914, H.G. Wells wrote a series of essays, "The War That Will End War," which advocated for peace and the disarmament of the German Empire. As the book rose in popularity, the title soon became both a slogan and rallying cry. 

Germany, at the time of WWI, had one of the world's most advanced chemical industries, and attempted to weaponize this as early as 1914.

Ypres, Belgium, was the site of the very first chemical attack, on April 22, 1915

The US saw 323,018 deaths in WWI, per Britannica.

Even opposing sides could sometimes unite as humans during the tough fights, with soldiers seldom looking out for each other.

Liverpool was home to a rest camp for American troops.

By the end of the war, many soldiers returned to their families. Thousands of others, however, weren't so fortunate.

A much larger effort, more than 12% of the US population served in World War II.

Since the end of the Vietnam War, America has employed an all-volunteer force, The New York Times reported.

A formal fitness test was introduced by the US Army in 1942. The test reportedly included push-ups, sit-ups, pull-ups, a 300-yard run, and squat jumps.

World War II involved more than 30 countries and lasted for six years. An estimated 85 million people were killed, according to The National WWII Museum.

The end of the war marked the beginning of the Baby Boom as thousands of soldiers came home and formed families.

In 1973, the US decided to end the citizen-soldier tradition and instead establish a large, professional all-volunteer force. Today, less than 0.5% of the population serves, according to the Council on Foreign Relations.

The US is the world's most powerful military, according to firepower, but trails behind countries like South Korea, North Korea, India, China, and Russia in terms of size.

Countries around the world still have military service, like Brazil, Turkey, Sweden, and Israel, to name a few.

Israel's military is often regarded as one of the most technologically advanced today.

The US' defense spending is the highest in the world.

While military history is vast and deadly, the effects of soldiers' efforts can be seen everywhere in today's political climate.