Frida Ghitis, a former CNN producer and correspondent, is a world affairs columnist. She is a frequent opinion contributor to CNN and The Washington Post and a columnist for World Politics Review. The opinions expressed in this commentary are her own.

(CNN) It should come as no surprise that North Korea has apparently just slammed the brakes on the peace train, which until a few hours ago seemed to be chugging happily along, on its way to a historic meeting between President Donald Trump and North Korean leader Kim Jong Un next month in Singapore.

The meeting is not canceled, but the North Koreans shocked Washington and Seoul when they announced at the last moment that talks with South Korea, scheduled for Wednesday, were "indefinitely" postponed because of long-planned military exercises between the United States and South Korea.

The announcement from the North's official news agency came with a message for Trump -- a threat, really -- that the United States, should "give serious thought" to the possible impact of the military exercises on the Kim-Trump summit, warning, "We will be closely watching the attitude of the United States and South Korean authorities."

Hours later, North Korea went much further, saying it may cancel the summit altogether over US demands that it abandon its nuclear weapons. That demand, to be clear, is precisely what Trump -- and his predecessors -- have sought all along.

The jolting announcement is a test for Trump, who, despite his claim to great dealmaking prowess, has already committed one of the most basic mistakes of any negotiator. Trump tries to act nonchalant, but it's no secret that a deal with North Korea would amount to a huge political win for an embattled President, who is already fantasizing about winning a Nobel Peace Prize. Just last week, when asked if he deserves the Nobel, he beamed, " Everyone thinks so ."

Kim is testing Trump. He is trying to do precisely what his father and grandfather before him managed to pull off: extract concessions, economic and political gains, while making small concessions or promises they later failed to keep.

Pyongyang's sudden indignation at this week's US-South Korea exercises, known as "Max Thunder," is a sham. The annual exercises are aimed at maintaining military coordination between the United States and its ally, to remain ready in case of a North Korean attack. After all, North Korea has attacked in the past.

Pyongyang knew Max Thunder was coming. In fact, last March, shortly after visiting Pyongyang, the South Korean national security adviser said in a statement that "[Kim] understands that the routine joint military exercises between the Republic of Korea and the United States must continue. And he expressed his eagerness to meet President Trump as soon as possible."

Kim is trying to find out just how much Trump is willing to do to prevent the collapse of the summit.

Not long after the surprise announcement, the Pentagon appeared to suggest it will not budge. We'll see how Trump responds.

Giving in to Kim on the exercises would constitute another concession in what are already lopsided pre-summit negotiations. North Korea has always wanted the military exercises to end. That should be an item for negotiation, not a concession ahead of the talks.

Already, Trump, whose administration has lost its top nuclear and North Korean experts, has given Kim much of what he wanted. For decades North Korea wanted a top-level meeting with US officials. Trump has not only granted it in exchange for not very much but has also been praising Kim, calling him " very honorable " and " really excellent ."

All the bad feelings seem forgotten, even though Kim runs a brutal regime, rife with human rights violations from enslavement to murder, according to human rights groups and the United Nations.

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So far, Kim's main concession is a promise to dismantle a nuclear testing site, but many experts say the site is damaged from previous explosions and has already served its purpose, helping complete North Korea's nuclear arsenal.

Trump has mocked his predecessors for making concessions that ultimately produced no results. He has vowed that he will accept nothing but a North Korea without nuclear weapons. Kim is testing him, and the world is watching.

Editor's note: This commentary has been updated to reflect news of North Korea's further comments Tuesday.