RUTHFULLY YOURS

Mr. Trump chose not to withdraw from the nuclear deal despite his ferocious criticism during the campaign and again on Friday. The deal itself is a piece of paper that Mr. Obama signed at the United Nations but never submitted to Congress as a treaty. The certification is an obligation of American law, the Iran Nuclear Review Act of 2015, that requires a President to report every 90 days whether Iran is complying with the deal. Mr. Trump said Iran isn’t “living up to the spirit of the deal” and he listed “multiple violations.”

The President can thus say he’s honoring his campaign opposition to the pact, without taking responsibility for blowing it up. This partial punt is a bow to the Europeans and some of his own advisers who fear the consequences if the U.S. withdraws. The worry is that Iran could use that as an excuse to walk away itself, and sprint to build a bomb, while the U.S. would be unable to reimpose the global sanctions that drove Iran to negotiate.

This is unlikely because the deal is so advantageous for Iran. The ruling mullahs need the foreign investment the deal allows, and there are enough holes to let Iran do research and break out once the deal begins phasing out in 2025. Iran will huff and puff about Mr. Trump’s decertification, but it wants the deal intact.

Yet we can understand why Mr. Trump wants to avoid an immediate break with European leaders who like the deal. This gives the U.S. time to persuade Europe of ways to strengthen the accord. French President Emmanuel Macron has talked publicly about dealing with Iran’s ballistic missile threat, and a joint statement by British, German and French leaders Friday left room to address Iranian aggression.

Meanwhile, Mr. Trump is asking Congress to rewrite the Nuclear Review Act to set new “red lines” on Iranian behavior. The Administration has been working for months with GOP Senators Bob Corker (Tenn.) and Tom Cotton (Ark.) on legislation they’ll unveil as early as next week. This will include markers such as limits on ballistic missiles and centrifuges and ending the deal’s sunset provisions. If Iran crosses those lines, the pre-deal sanctions would snap back on.

There’s no guarantee this can get 60 Senate votes. But making Iran’s behavior the trigger for snap-back sanctions is what Mr. Obama also said he favored while he was selling the deal in 2015. The difference is that once he signed the deal his Administration had no incentive to enforce it lest he concede a mistake. The Senate legislation would make snap-back sanctions a more realistic discipline. Senators may also want to act to deter Mr. Trump from totally withdrawing sometime in the future—as he threatened Friday if Congress fails.

The most promising part of Mr. Trump’s strategy is its vow to deter Iranian imperialism in the Middle East. The President laid out a long history of Iran’s depredations—such as backing for Syrian dictator Bashar Assad and rebels in Yemen, cyber attacks on the U.S., hostility to Israel, and support for terrorism. Notably, Mr. Trump singled out the Islamic Revolutionary Guard Corps, the regime’s military vanguard, for new U.S. financial sanctions.

This is a welcome change from President Obama, who was so preoccupied with getting his nuclear deal that he ignored Iran’s efforts to expand the Shiite Islamic revolution. Mr. Trump is putting the nuclear issue in the proper strategic context as merely one part of the larger Iranian attempt to dominate the region. This will go down well with Israel and the Sunni Arab states that were horrified by Mr. Obama’s tilt toward Tehran.

One question is how this squares with Mr. Trump’s cease-fire deal with Russia in southern Syria. Russia is allied with Iran in Syria, and the cease-fire is serving as protection for Revolutionary Guard attempts to control the border region with Israel, which has had to bomb the area repeatedly. Mr. Trump still hasn’t figured out a strategy for Syria or Russia, and that could undermine his effort to contain Iran.

Barack Obama left his successor a world in turmoil, with authoritarians on the march in China, North Korea, Russia and Iran. Mr. Trump needs a strategy for each, and the steps he took Friday are crucial in containing Iran.