US President Donald Trump has canceled his summit with North Korean leader Kim Jong-un scheduled for June 12 in Singapore. More precisely, the summit has been postponed indefinitely.

It is not clear yet exactly what caused the abrupt U-turn.

In view of the open letter Trump wrote to Kim, it appears that the decision was prompted by North Korea’s First Vice Foreign Minister Kim Kye-gwan’s criticism of US national security adviser John Bolton, followed by North Korean Vice Foreign Minister Choe Son-hui’s nuclear war threats and rude remarks regarding US Vice President Mike Pence.

It is commonplace for North Korea to criticize the other side bluntly ahead of an important negotiation, but Trump, unfamiliar with such behavior, may have found it insulting.

On the surface, Trump raised issues with the North’s hostile attitude, but probably the two sides could not narrow their differences over the dismantlement of North Korea’s nuclear program.

Trump hinted his willingness to accept the phased denuclearization that the North demanded, on the condition that it must be done as quickly as possible, but the North may have insisted on compensation in as many stages as possible as it did in the past. Trump has vowed not to repeat past mistakes of allowing such denuclearization tactics.

North Korea’s recent escalation of pressure on the South also played some role in fostering Trump’s suspicions of Kim’s commitment to denuclearize his country.

Pyongyang abruptly canceled inter-Korean high-level talks on May 16, initially refused to allow South Korean journalists to cover the demolition of the Punggye-ri nuclear test site on May 18 -- though it later allowed them to -- and demanded on May 19 that the South repatriate defected North Korean waitresses.

Skepticism of the Trump-Kim summit grew in the US over Kim’s show of opposition to Bolton’s way of denuclearization after his surprise summit with Chinese President Xi Jinping.

Even though the North warned of the collapse of the summit through statements by Kim and Choe, it blasted the nuclear test site as it had pledged.

On one hand, it got on the nerves of the US, and on the other, it showed its commitment to denuclearization.

After blowing up the site, a North Korean nuclear weapons institute said in a statement that the nuclear test suspension was an important step for a global nuclear arms reduction. It is understood to mean that the communist state will link future negotiations on its denuclearization to the reduction of US forces in South Korea and the prevention of the US deployment of strategic military assets.

This arouses suspicions that the North is not as committed as it pledged to accomplish complete, verifiable, irreversible denuclearization demanded by the US. North Korea aroused suspicions of its commitment by refusing to allow experts to inspect the demolition of the nuclear test site.

Naturally nerves can happen ahead of a crucial negotiation, but the North failed to abandon the brinkmanship and intimidation tactics it has used for the past 25 years, such as treading the tricky path between Washington and Beijing and using coarse language to gain an advantage in negotiations. They did not work with Trump, though, and eventually have caused a grave crisis to denuclearization efforts so far.

The US and the North were supposed to make working contact in Singapore this weekend. The announcement of the summit’s cancellation at this critical moment must be a bolt from the blue to the South Korean government, which has tried hard to broker the Trump-Kim meeting successfully.

Nevertheless, we look forward to seeing a grand deal come at the last minute. Seoul must resume brokering, and Pyongyang must refrain from tit-for-tat provocative words and deeds.

Presently, the June 12 summit between the US and North Korea looks derailed, but the door is not tightly closed yet as Trump has left open the possibility of future negotiations with Kim. The ball is in Kim’s court. To give up its nuclear arsenal without doubt is the only way for the North to achieve co-prosperity with the South. It is time for Kim to make a decision for the completely denuclearized future.