More than 100 AI experts have written to the UN asking them to ban lethal autonomous weapons — those that use AI to act independently. No so-called "killer robots" currently exist, but advances in artificial intelligence have made them a real possibility. Experts said these weapons could be "the third revolution in warfare," after gunpowder and nuclear arms.

The "first revolution in warfare" was invented by the Chinese, who started using the explosive black substance between the 10th and 12th centuries to propel projectiles in simple guns. It gradually spread to the Middle East and Europe in the following two centuries. Once perfected, firearms using gunpowder proved to be far more lethal than the traditional bow and arrow.

The invention of gunpowder also introduced artillery pieces to the battlefield. Armies started using basic cannons in the 16th century to fire heavy metal balls at opposing infantrymen and breach defensive walls around cities and fortresses. Far more destructive field guns were invented in the 19th century and went on to wreak havoc in the battlefields of World War I.

Guns that fire multiple rounds in rapid succession were invented in the late 19th century and immediately transformed the battlefield. Machine guns, as they came to be known, allowed soldiers to mow down the enemy from a protected position. The weapon's grisly effectiveness became all too clear in WWI as both sides used machine guns to wipe out soldiers charging across no man's land.

Military thinkers did not ignore the invention of the first airplane in 1903. Six years later, the US military bought the first unarmed military aircraft, the 1909 Wright Military Flyer. Inventors experimented with more advanced fighter and bomber aircraft in the following years. Both became standard features in many of the national air forces established by the end of WWI.

Armies had traditionally used soldiers and horses to fight and transport military equipment. But around WWI, they started using more machines such as tanks and armored vehicles. Faster and more destructive armies were the result. Nazi Germany put this new form of "mechanized warfare" to destructive effect in WWII using an attack strategy known as "Blitzkrieg" ("lightning war").

Although artillery was effective, it had a relatively limited range. The missile's invention in WWII suddenly allowed an army to strike a target hundreds of kilometers away. The first missile — the German V-2 — was relatively primitive, but it laid the foundation for the development of guided cruise missiles and intercontinental ballistic missiles (ICBM) capable of carrying nuclear warheads.

Jet aircraft first saw action alongside traditional propeller airplanes at the end of WWII. Jet engines dramatically increased an aircraft's speed, allowing it to reach a target quicker and making it far harder for an adversary to shoot it down. After WWII, military reconnaissance planes were developed that could fly higher than 25 kilometers (15.5 miles) and faster than the speed of sound.

The "second revolution in warfare" announced its horrific arrival on August 6, 1945, when the US dropped the first nuclear bomb — "Little Boy" — on the city of Hiroshima in Japan, killing between 60,000 and 80,000 people instantly. In the Cold War that followed, the US and Soviet Union developed thousands of even more destructive warheads that raised the specter of a devastating nuclear war.

Recent decades have witnessed the ever more prevalent use of computers to conduct war. The devices made military communication quicker and easier and radically improved the precision and efficiency of many weapons. Armed forces have recently focused on developing cyber warfare capabilities to defend national infrastructure and attack foreign adversaries in cyberspace.