With the North Korean denuclearization process sputtering to a halt before it made any headway, Pyongyang and Washington are returning to familiar tactics, which had been largely suspended since the summit between their leaders in Singapore in June.

Now both sides are talking up tensions.

US Secretary of Defense James Mattis hinted that South Korea and the United States could resume military exercises – halted as a goodwill gesture by US President Donald Trump in June after his summit with North Korean leader Kim Jong Un – soon after a senior North Korean official reportedly sent a belligerent letter to the White House.

Threats in word and in letter

Speaking at a media briefing at the Pentagon on Tuesday, Mattis said: “We took the step to suspend several of the largest exercises as a good-faith measure coming out of the Singapore summit. We have no plans at this time to suspend any more exercises.”

North Korea consistently complains that large-scale annual joint exercises, which usually take place in the spring and summer, are preparations to invade. In response, it ramps up its own rhetoric, raising tensions across Northeast Asia.

Mattis noted that small exercises are constantly ongoing in South Korea. “The reason you’ve not heard much about them is [so] North Korea could not in any way misinterpret those as somehow breaking faith with the negotiation,” he said.

Even so, Mattis noted that diplomacy remained the core focus of US policy, de-emphasizing a military role. “We stay in a supporting role,” he said.

His words echoed those of the senior US soldiers in South Korea. General Vincent Brooks told reporters last week that the military role “has been to adjust our operations and exercises in such a way that if we want to increase pressure, we can do that, and if we want to decrease pressure, we can do that.”

“We have been told for now to put the sword in its sheath – but we have not been told to forget how to use it.”

Mattis’ comments came after it was reported by US media, including CNN and the Washington Post, that a belligerent letter had been sent from Kim Yong Chol, the former general of the North Korean espionage bureau who is now a close aide to Kim Jong Un and a key player in the denuclearization talks, to US Secretary of State Mike Pompeo.

According to unnamed sources cited by CNN, the letter said that denuclearization talks “may fall apart” and if a compromise cannot be reached, Pyongyang could resume “nuclear and missile activities.”

Denuclearization stalled

Trump abruptly cancelled Pompeo’s latest visit to Pyongyang on Friday. The cancellation came hours before he was due to depart, in the company of newly appointed special envoy on North Korea Stephen Biegun. At the time, Trump cited lack of progress on denuclearization as the reason.

Various reports and satellite image analyses indicate that work continues at North Korean nuclear and missile facilities, while moves to dismantle a missile-engine test site – a move that Kim promised Trump in Singapore – have apparently been halted.

Pompeo’s previous trip to Pyongyang, and his first since the Singapore summit, had not gone well. Pompeo, who seeks a full list of North Korean nuclear sites and assets as a first step in a mutually-agreed upon denuclearization process, returned home empty-handed, and North Korean state media criticized him for “gangster-like” tactics.

The United States’ position is that rewards, such as sanctions relief and the opening of formal diplomatic relations, should follow denuclearization. North Korea, on the other hand, seeks a phased approach of concession for concession.

Pyongyang state media has recently been pressing for a peace treaty to replace the armistice that formally ended the 1950-53 Korean War. Washington has not moved on the issue, apparently considering it leverage to be kept in hand.

Moon’s mission in September

Even so, North Korean state media has not criticized Trump himself, and Trump continues to speak of Kim with respect. South Korean President Moon Jae-in is expected to travel to Pyongyang for a summit with Kim next month, in the hope of playing an intermediary role between North Korea and the US.

“If we say the denuclearization process is in a deadlock, that will only be another reason to hold a South-North summit,” a South Korean presidential official said, according to Yonhap news agency. “As I have said on several occasions, we believe development in the US-North Korea relationship should promote development in the South-North Korean relationship and development in inter-Korean relations must help lead improvements in the US-North Korea relationship,” a presidential spokesman added.

But however Seoul paints it, the challenge facing Moon, as he seeks to bridge the chasm between North Korea and the United States, is immense.

“This puts Moon in an extremely unpleasant situation,” said Andrei Lankov, a North Korea expert at Kookmin University. “For Moon and South Korea, the major short-term security threat is the US, not North Korea: If, tomorrow, shells start to explode in Seoul, it will not be because North Korea has decided to take over the South, it will be because Donald Trump decided to start a ‘bloody nose’ operation.”

Even so, the Seoul-based Russian expects calm to prevail in Korea for the next few months as Trump focuses on the upcoming mid-term elections in November. But he admits there are risks.

“The question is whether Trump will decide to go bellicose again, or just keep the fiction of the ongoing denuclearization  process alive as long as possible,” he said. “If his failure becomes too obvious, he might feel obliged to react and then we would return to a highly risky situation.”