Sandwiched between South Korea proposing talks with North Korea Tuesday, and Pyongyang responding with a call to Seoul a day later, Donald Trump posted a tweet threatening to start nuclear war on the Korean Peninsula.

The hotline between the North and South, dormant since December 2015 when Pyongyang ordered it closed, rang at 3.30 p.m. local time Wednesday (1.30 a.m ET) in the shared border village of Panmunjom.

A discussion took place about sending a North Korean delegation to the Winter Games in Pyeongchang in February.

South Korea confirmed it received the call, labelling the move “very significant” for creating “an environment where communication will be possible at all times.”

The return of communications between the countries was first suggested Monday by Kim in his annual New Year’s Day address.

Yet hours into the unexpected diplomatic thaw, Trump signaled the U.S. had little interest in easing tensions, posting how he has a “bigger and more powerful” nuclear button than the North Korean leader.

Trump, recently back from several days golfing in Florida, posted his tweet in response to Kim’s claim Monday that his nuclear forces are “completed” and “the button for nuclear weapons is on my desk.”

Nikki Haley, the U.S. ambassador to the U.N., also moved to undermine the burgeoning detente, saying the U.S. would not take talks seriously if Pyongyang failed to abandon its nuclear arsenal.

Trump’s tweet was widely condemned by national security experts and many Democrats, including Rep. Eric Sewell who appealed for help to “put this lunacy in check.”

Rep. Jim Himes told CNN that Trump’s tweet has “Freudians” abuzz and shows an impulse “to demonstrate that his is bigger and stronger than anybody else’s.”