Not even the fate of American student Otto Warmbier, who died this week after returning to the United States following his detainment in North Korea, will dissuade advocates of “engagement” with Pyongyang. They argue that, however repugnant the regime, diplomacy is the only way to stop North Korea’s rapidly advancing nuclear and missile programs. But our recent experience suggests that trying to talk to supreme leader Kim Jong Un is a waste of time.

This month, we were part of a group of delegates from the United States, Japan, China and South Korea who met in Sweden with representatives of North Korea to explore possible grounds for resuming the six-party talksthat collapsed in 2009. After many hours with the North Korean delegation at these “1.5 track” talks, we left more pessimistic than when we arrived.

North Korean officials made unambiguously clear that Pyongyang will not be deterred from augmenting its nuclear arsenal or test-launching an intercontinental ballistic missile that could eventually threaten the U.S. homeland. There were no signals of flexibility or willingness to negotiate on these programs.

Throughout, the North Korean message was that denuclearization is off the table. Pyongyang’s representatives declared: “The most perfect weapons system will never become the exclusive property of the United States.”

We tried repeatedly to ascertain whether any combination of economic and diplomatic benefits or security reassurances could induce Pyongyang to comply with its previously negotiated commitments and with U.N. resolutions. The answer was an emphatic, unwavering no. Citing the fates of Iraq’s Saddam Hussein and Libya’s Moammar Gaddafi, the North Koreans repeatedly said that their nuclear program is the ultimate life preserver for the regime.

Our North Korean interlocutors presented a stark choice: “First accept us as a nuclear state, then we are prepared to talk about a peace treaty or fight. We are ready for either.” The North Koreans weren’t saying that they would initiate hostilities but that they would fight if provoked. A peace treaty ending the Korean War and legitimizing the North Korean state is a long- standing goal for Pyongyang, which sees it as a catalyst for the removal of all U.S. forces from the peninsula.

Strikingly different from similar meetings in the past was the self-confidence, even cockiness, of the North Koreans, clearly a result of the recent successes of their nuclear and missile programs. The North Koreans also made clear that their nuclear program is a response to the general “U.S. hostile policy.” As such, nothing Seoul could offer would alter Pyongyang’s commitment to its nuclear arsenal. The North Koreans won’t even deign to negotiate with the South Koreans, whom they described repeatedly as “puppets” of the United States. Thus, the new South Korean president, Moon Jae-in, is in for a major disappointment if he tries to resurrect the “sunshine policy” of unconditional engagement pursued by previous progressive presidents from 1998 to 2008.

President Trump has placed his hopes on Chinese promises to more fully implement U.N. sanctions. But as even he now seems to acknowledge, this hasn’t happened. He tweeted on Tuesday: “While I greatly appreciate the efforts of President Xi [Jinping] & China to help with North Korea, it has not worked out. At least I know China tried!”

Although Trump has criticized President Barack Obama’s “strategic patience” policy as weak and ineffectual, he has yet to distinguish his North Korea policy from his predecessor’s. Trump’s policy of “maximum pressure” is anything but, and he continues to pull his punches against North Korean and Chinese violators of U.S. law. The Trump administration has also sent conflicting signals about whether it would negotiate with North Korea or potentially conduct a military attack to prevent the regime from mastering an intercontinental ballistic missile.

Initiating a preemptive strike would be a bad idea against a state that already has nuclear weapons, as well as 10,000 artillery tubes aimed at Seoul. In our talks, the North Korean officials emphasized that they did not struggle to acquire nuclear weapons only to perish without using them. The implied threat was clear: If the United States were to use military force against North Korea, Pyongyang would retaliate, potentially leading to hundreds of thousands or millions of casualties.

Instead of trying to preempt the North Korean nuclear weapons program, the Trump administration would be better advised to ramp up sanctions — including secondary sanctions, despite predictable Chinese protests. This would impose a penalty on North Korea, without risking a war — and could conceivably hasten the day the Kim regime finally collapses. Bolstering sanctions might not be exciting, but it would be a more pragmatic step than yet another attempt at negotiations.