These subjects, already discussed at lower levels, pose little risk that lunch in Helsinki will spiral out of control. Nuclear policy may offer a similar opportunity. Both Mr. Trump and Mr. Putin have spoken lately about managing “the arms race.” They can safely instruct their advisers to keep discussing it after the summit.

The second requirement for the success of the Singapore model is to get Mr. Putin to take it seriously. He has to see Helsinki as, at most, the initial step in a new relationship — one that can develop beyond high-level joviality only if Moscow changes course. Unfortunately, Mr. Trump, having said so often how much he wants to “get along,” gives Mr. Putin few incentives to rethink. Despite frequently poor results (Ukraine is the outstanding case), Russian policy has not shifted on any major issue. And why should it? An American president who touts his own unanchored, unpredictable views lets Russian policymakers hope they can have what they want for nothing.

Mr. Trump’s advisers are probably too embarrassed to tell him how many people in Moscow describe him as an easy mark for manipulation. They can’t stop snarky Russians from saying the president of the United States can be duped. But if, at Helsinki and after, Mr. Pompeo and Mr. Bolton show Mr. Putin that they can’t be duped and that their boss does what they say, they will have taken a long step toward a more productive Russian-American relationship.

Finally, if Mr. Trump and his advisers want to “get along” with Moscow in a way that serves American interests, they have to prove they can deliver. From Mr. Putin on down, Russian officials keep asking why Mr. Trump has been so powerless to implement his own preferred policy. It’s a fair question, one the president must ask himself. The answer is not some “deep state” conspiracy against him. It’s that neither the White House nor the Kremlin understands the American policy process.

Almost no one — not even the president — takes American foreign policy in a new direction all alone. Mr. Trump has been no exception. Last summer, a near-unanimous Congress sharply curtailed his ability to offer Moscow relief from sanctions on his own. The Pentagon, the C.I.A. and the Treasury remain powerful dissenting voices on other issues involving Russia.

Even our NATO allies have a say. Mr. Putin probably hoped that before going to Helsinki, Mr. Trump would disrupt the NATO summit in Brussels as thoroughly as he did the Group of 7 summit in Canada. The president, from his wild opening rant about German purchase of Russian gas, certainly obliged him. But the result may not be what Mr. Putin expects. Chaos in the Western alliance has already sent shock waves through all the institutions that shape American policy. These shocks incite resistance. They may make it harder for Mr. Trump to impose his will.

All these prerequisites for transforming Russian-American relations converge on a single point that must frustrate both Mr. Trump and Mr. Putin. The American president cannot decide everything by himself. If Mr. Trump wants to succeed, he needs to defer more to those — from tough-minded advisers, to congressional antagonists, to angry allies — who disagree with him. If he tries to control the process without answering their concerns, he will continue to fail.