President Donald Trump landed back in the United States Thursday after a surprise directive to begin nuclear weapons testing that raised the specter of renewed superpower tensions.

The announcement on social media was issued right as Trump was going into a summit with Chinese leader Xi Jinping in South Korea.

And it came days after Russia declared it had tested a nuclear-capable cruise missile and a nuclear-powered, nuclear-capable sea drone.

The blunt statement from Trump, who boasts frequently about being a "peace" president, left much unanswered.

Chiefly, it was unclear whether he meant testing weapons systems or actually conducting test explosions -- something the United States has not done since 1992.

Regardless, the statement amounted to unusual nuclear sabre rattling.

"Because

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