One day after Rex Tillerson announced a minor diplomatic breakthrough in the North Korea crisis, saying that the U.S. had “lines of communication” open to the Kim regime, President Donald Trump swiftly undercut his embattled secretary of state, tweeting that his efforts were pointless and suggesting that he would handle the problem militarily. “I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man . . . Save your energy Rex, we’ll do what has to be done!” the president tweeted, in reference to North Korean leader Kim Jong Un.

R.C. Hammond, the spokesman for the State Department, was quick to downplay his boss’s remarks, dismissing the idea that substantive talks were underway between the United States and North Korea. “There is a means by which the countries can engage with each other,” Hammond told reporters. “North Korean officials show no indication that they are interested in or ready for talks on denuclearization.” He also cast Trump’s tweet as a message to North Korea—not Tillerson, whose poor relationship with the White House is an open secret in Washington. “The President just sent a clear message to NK: show up at the diplomatic table before the invitation gets cold,” he wrote.

But among the diplomatic community, Trump’s tweet was rightly interpreted as a stunning rebuke of a Cabinet member—and a portent of an early exit for the secretary. “Humiliating for Tillerson, but worse, renders him useless,” Dan Shapiro, the former U.S. ambassador to Israel under Barack Obama, wrote on Twitter. “He’ll resign, today or after a brief face-saving interval.” Michael McFaul, who previously served as the U.S. ambassador to Russia, floated the possibility that Trump’s tweets were part of a broader “good cop, bad cop” strategy on North Korea, but added, “I fear it’s not.” Vali Nasr, a former senior adviser to the U.S. special representative for Afghanistan and Pakistan, characterized the tweet as “astonishing advice from a president undermining his chief diplomat trying to defuse a crisis.”

Trump’s erratic behavior may be tactical; as Axios reported Sunday, the president has apparently instructed aides to portray him as “crazy” in negotiations. But there are limits to the effectiveness of the madman approach, as foreign-affairs experts have repeatedly warned—especially in North Korea, where officials have said that threats of force will only cause Kim Jong Un to intensify his efforts to build a credible nuclear deterrent. Without the backing of the president, it is not clear what, if anything, Tillerson can now do or say that will be seen as legitimate as the State Department struggles to avoid a war the White House appears to be hastening.

During an interview with NBC’s Meet the Press on Sunday, Senator Bob Corker, the Republican chairman of the Senate Foreign Relations Committee, stressed the importance of diplomacy in the face of the growing nuclear threat. “If we don’t ramp up the diplomatic side . . . it’s possible that we end up cornered.” The Tennessee lawmaker who announced his retirement last week also told host Chuck Todd that he stands by the comments he made about the president in August, in which he called for “radical changes” in the White House following the violence in Charlottesville.

Shortly after Corker’s appearance, the president doubled down on his earlier tweet, suggesting again that the U.S. had exhausted its diplomatic options with North Korea. “Being nice to Rocket Man hasn’t worked in 25 years, why would it work now? Clinton failed, Bush failed, and Obama failed. I won’t fail,” he wrote.