Diplomats should not be confused with donkeys, but the carrot and the stick are useful tools to encourage both diplomat and donkey to do the right thing. President Trump has aggressively wielded these implements to deal with rogue regimes in Iran and North Korea. The sight of three Americans freed from North Korean prison returning to American soil is evidence that change, limited but real, is at hand.

North Korea is front and center of crisis time.

In that other cockpit of crisis, Mr. Trump put the cat among the pigeons of the Middle East with the American withdrawal from the Iran Nuclear Deal. No one thinks the deal was a good one but the diplomats of the West have persuaded themselves that there’s nothing anybody can do about it, and everybody should learn to live with it.

The Iran nuclear deal was a sham from Day One. The Iranians never bothered to sign it. As Israeli Prime Minister Benjamin Netanyahu revealed in his disclosure of seized Iranian documents, the mullahs have been lying for years to hide their quest for nuclear weapons.

Mr. Trump’s critics argue that his rough application of the stick to Iran will persuade the similarly hostile regime in North Korea that the United States can’t be trusted to honor its agreements. The opposite is more likely true. The president has been entirely up-front with Kim Jong-un, backing his warnings to him with repeated displays of military might. Canceling the deal with Iran sends the clear message that there will be no place for guile at the negotiating table.

If Mr. Trump can fashion a constructive relationship with Mr. Kim, he will demonstrate to the mullahs of Iran that hostility doesn’t have to be forever.