The Trump team, led on alliance issues by Defense Secretary Jim Mattis, is poised to have a successful NATO summit if the president can take “yes” for an answer. The combined defense budget of NATO nations has grown by $14.4 billion since Mr. Trump took office (increases began under Barack Obama). All but one of 28 allies are increasing spending, and 26 are sending more troops to NATO missions. Sixteen are on track to spend 2 percent of their gross domestic product on defense by 2024, NATO’s target.

Rather than thrash allies, the president should celebrate this success, take credit for it, and accelerate bilateral work to help close remaining spending gaps. Other NATO achievements worth celebrating include two new military commands that will increase the readiness of alliance forces and speed deployments. These moves, directed against any further territorial ambitions Moscow may have, should strengthen Mr. Trump’s hand at Helsinki.

The leverage NATO gives Mr. Trump at the Putin summit will be wasted, however, if the message from Brussels mirrors the president’s presentation at the Group of 7 meeting last month: Allies are feckless free-riders, America doesn’t need them and it’s the planet’s autocrats who deserve our respect.

Mr. Putin, the biggest winner from any disunity in NATO, is counting on the second outcome. The only additional thing he needs to make his Helsinki meeting a success is money. Here, Mr. Trump is holding a hand nearly as strong as Ronald Reagan’s in 1982 — if he plays it right. Mr. Putin survives on a governance model that requires $60-per-barrel oil, total political control of his citizenry and a kleptocratic stranglehold on the economy. The reform Russia needs is impossible without more power-sharing than he will allow.