Which countries have nuclear weapons and how are they tested?

President Donald Trump's comments Thursday suggesting the United States will restart its testing of nuclear weapons upends decades of American policy in regards to the bomb, but come as Washington's rivals have been expanding and testing their nuclear-capable arsenals.

Nuclear weapons policy, once thought to be a relic of the Cold War, increasingly has come to the fore as Russia has made repeated atomic threats to both the U.S. and Europe during its war on Ukraine. Moscow also acknowledged this week testing a nuclear-powered-and-capable cruise missile called the Burevestnik, code-named Skyfall by NATO, and a nuclear-armed underwater drone.

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China is building more ground-based nuclear missile silos. Meanwhile, Nort