The United States has officially destroyed its stockpile of chemical weapons.

In doing so, it is the last of eight countries to destroy its declared stocks of chemical weapons under the Chemical Weapons Convention – an international treaty signed by nearly 200 nations that bans the possession, production and use of the weapons on the battlefield.

The milestone was reached on Friday at the Bluegrass Chemical Agent-Destruction Pilot Plant in Kentucky. A final rocket was drained of sarin nerve agent, and that agent was then chemically deactivated and destroyed.

The destruction of that single rocket means that "one hundred percent of the world's declared chemical weapons have now been destroyed," says Kingston Reif, the Deputy Assistant Secretary of Defense for Threat Reduction and Arms Control, who oversaw the process.

Here's why the world needed that ban in the first place.

Who had chemical weapons?

Chemical weapons were used to terrible effect during World War I. More than 1.3 million soldiers were exposed to chemical weapons during the Great War and 100,000 died directly as a result of chemical attacks.

During World War II, nations around the world stocked up on chemical weapons, in case they were used again. They were never deployed on the battlefield, though Nazi forces used poisonous gas to murder millions of people in concentration camps.

During the Cold War, the U.S. and Russia stockpiled vast quantities of chemical weapons. By 1990, the U.S. had more than 30,000 tons of chemical agents, and Russia likely had at least 40,000 tons, according to David Koplow, a professor of law at Georgetown University in Washington, DC.

In 1997, the U.S. Senate ratified the Chemical Weapons Convention. "The convention requires other nations to follow our lead," then President Bill Clinton said at the time.

It turns out the U.S. did not stay in the lead. In fact, it ended up being the last nation to eliminate its chemical weapons stockpile.

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Why did it take so long for the U.S. to eliminate their weapons?

The weapons themselves were tricky to handle, says Koplow. They contained deadly chemicals and explosives crammed into an artillery shell, rocket or bomb. They weren't designed to be taken apart, and many were manufactured decades ago, making them fragile and unreliable.

Then there was the disposal process itself. The Army's practice was to incinerate these agents — but at some sites, locals resisted, fearing dangerous pollution, so entirely new techniques had to be developed. Mustard agent, for example, was broken down by bacteria, and sarin was chemically deactivated before it was destroyed.

But that wasn't the whole story. Koplow says the program was also plagued by under funding and poor management. "The leadership for the program changed repeatedly and it was just never taken as seriously as it should have been," he says.

As a result it dragged on for so long that the U.S. was actually in violation of the treaty for several years. But that finally ended on Friday.

Has the chemical weapons convention worked overall?

There are still nations who have used covertly produced chemical weapons in recent years. Most notably, Syria deployed chlorine and nerve agents in its civil war with horrible effects. Russia has used some chemicals for targeted assassination attempts, and North Korea's leader Kim Jong Un used nerve agent to kill his half brother.

But those are isolated cases. More broadly, vast quantities of chemical weapons have been disposed of by nations all over the world. And Reif says that overall that's something to celebrate.

"These are awful weapons," he says. "The world is a safer and more secure place without them."

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