Story highlights Nine nuclear-capable states exist in the world, experts say

Eight believed to have weapons ready to be incorporated into their arsenals

(CNN) Nine nuclear-capable states exist in the world, but only eight of them -- all but North Korea -- are believed by experts to have weapons mature and developed enough to be readily incorporated into the arsenals of their armed forces.

That is, of course, a rapidly developing situation. A North Korean official threatened this week that his country would test a hydrogen bomb over the Pacific Ocean after President Donald Trump called North Korean leader Kim Jong Un "rocket man" and the North Korean shot back that Trump was a "dotard ."

The road to North Korea's weapons capability follows decades of ups and downs as nuclear states such as the United States and Russia have built up and dismantled their programs.

Today, Russia is believed to have the most nuclear weapons, with 4,300. Not far behind is the United States, with 4,000, according to The Bulletin of the Atomic Scientists' Nuclear Notebook . Of the world's approximately 15,000 nuclear weapons, 9,400 are in military arsenals. The remaining weapons are retired and awaiting dismantlement. Nearly 4,000 are operationally available, and 1,800 are ready for use on short notice.

But after large-scale disarmament efforts by superpowers such as the United States, we're really at a low point in what was once a world that packed three or four times more operational nuclear power. See the chart below for a visualization of how nuclear arsenals have changed over time.

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