Throughout history, many brave individuals have been a part of the military, and have played pivotal roles in shaping society.
But while armies are as old as time, they too have changed over the years.
Take a look at these vintage photos of soldiers to see how much the military has evolved.
Militaries have existed since ancient times: The first war ever recorded took place in Mesopotamia in 2700 BCE.
Source: The Ancient History Encyclopedia
Despite the saying "all is fair in love and war," warfare usually abides by a certain etiquette.
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 honour their dead.
The French Revolution brought about the idea of conscription.
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.
Suddenly, militaries were no longer limited by the amount of soldiers they had, but by their country's population of able-bodied men.
Countries around the world still have conscription, and in some cases, military service is even getting reintroduced. Per BBC, Georgia reinstated military service in 2017, Lithuania reinstated conscription in 2016, and last year Sweden voted to reintroduce conscription as well.
France drafted its men during WWI, and eventually other European countries and the US followed suit.
In 1917, US Congress passed the Selective Service Act, which called for all men ages 21 to 30 in the 48 states plus Washington DC. In 1918, this expanded to include men from the ages of 18 to 45, and from Alaska, Hawaii, and Puerto Rico.
The United States military is made up of the Navy, National Guard, Coast Guard, Marines, Army and, most recently, the Air Force.
The Army, Navy and Marines were founded in 1775. The Coast Guard wasn't formed until 1790, and the Air Force wasn't formed until 1947.
When the US joined WWI, it had to get creative with how it trained its men, due to mass shortages of planes and weapons.
Pilots had to be trained with makeshift cockpits and imaginary guns.
Over 4.7 million men and women served in the US forces during World War I.
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.
With these huge new militaries came unthinkable death counts — there were an estimated 37 million casualties reported during World War I.
Source: Encyclopaedia Britannica.
People called it "the war to end all wars."
In 1914, H.G. Wells wrote a series of essays called "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.
The battles were grim — here, French soldiers wear makeshift gas masks in the trenches during the second battle of Ypres in 1915.
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
This photo shows wounded American soldiers by an ambulance in France.
But there were some moments of unity — here, a German soldier gives a wounded Russian a drink from his flask.
Here, US soldiers enjoy a small break after arriving in Liverpool in 1918.
Liverpool was home to a rest camp for American troops. According to Lewis Lukens, the Acting Ambassador to the United Kingdom, "A YMCA hut was set up on Lord Street. The American Hospital in Mossley Hill saw regular musical entertainment, and baseball games took place at Port Sunlight. Later, American public holidays were celebrated on Merseyside. In fact, between April 1917 and September 1918, more than 800,000 American men and women passed through the city of Liverpool - by far the busiest port in the UK to receive US troops and supporters."
When WWI ended in 1918, soldiers were finally able to return to their normal lives.
However, World War II brought even more civilians into combat — 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.
They trained rigorously in preparation — a formal fitness test was introduced by the US Army in 1942.
World War II was the biggest and deadliest war in history.
See the percentage of a country's population that died during WWII here.
After six years, it finally drew to an end — here, US soldiers hold a captured Nazi flag in a French village in 1944.
The number of people serving in the US armed forces has since dropped — now, less than 0.5% of the population serves.
In 1973, the US decided to end the citizen-soldier tradition and instead establish a large, professional all-volunteer force. Today, only 0.5% of the population serves.
The military's size varies from country to country.
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.
Some countries require military service from all its citizens, others don't.
Countries around the world still have military service, like Brazil, Turkey, Sweden, and Israel, to name a few.
Technological advancement also varies.
Israel is often regarded as having the most technologically advanced military today.
The United States is believed to be the world's most powerful military in terms of firepower.
Its defence spending is the highest in the world.
But no matter where or when they served, the sacrifices soldiers have made have changed the course of history.