If a typical American president were to meet with the Russian president, even in the context of the current deeply fraught relationship, it could be an opportunity to discuss important issues of mutual concern, to air differences and seek common ground. But, precisely because President Trump is anything but typical — including in that his campaign is under investigation for possibly coordinating with Russia to win the presidency and that he consistently lauds Vladimir V. Putin while denigrating our closest allies — his coming summit with Mr. Putin in Helsinki is a dangerous and counterproductive undertaking. The risks are many and the benefits, if any, are difficult to discern.

In normal circumstances, the American president would press Russia on multiple fronts. He would refresh demands that Russia: withdraw from Ukraine and renounce its illegal claim to Crimea; cease backing the murderous Assad regime in Syria and work for a diplomatic outcome that protects the rights and security of all Syrians; stop supporting the Taliban in Afghanistan; halt provocative military actions on NATO’s periphery and harassment of United States personnel in Moscow; extend the New Strategic Arms Reduction Treaty on nuclear weapons and come clean on its violation of the Intermediate Nuclear Forces treaty; press the North Korean dictator, Kim Jong-un, to denuclearize completely; cease destructive cyberoperations; and halt interference in America’s electoral processes and domestic politics, or face harsh additional sanctions. There is a rich and full agenda to pursue, if only we had a president who cared to advance American interests.

Moreover, if President Trump had a national security team willing and able to curb his reckless instincts, he would eschew the pageantry of a stand-alone summit with an avowed adversary and meet in a low-key fashion on the margins of another gathering, like the United Nations General Assembly or the G-20 meeting. The president, when under profound suspicion of being beholden in some manner to the Russians, should never be allowed to meet solo with the far more experienced, savvy and prepared Mr. Putin. This is a recipe for disaster, particularly since Mr. Trump has amply demonstrated his refusal to prepare for critical meetings, stick to a script, or avoid costly own-goals in his foreign engagements. The one-on-one format enables Russia to distort publicly the meeting’s substance and Mr. Trump to cede whatever he wants to Mr. Putin, with no note-taker or adviser present. Such a dialogue is not just diplomatic malpractice; in these unique circumstances, it amounts to rank insanity.

The Helsinki summit appears set to mark a watershed in President Trump’s comprehensive retreat from United States global leadership. In undermining NATO, launching a trade war against our closest allies, lauding authoritarianism, jettisoning concern for human rights and democratic values, and serially withdrawing from global commitments, Mr. Trump has already delivered to Mr. Putin much of the benefit he might have sought by supporting Mr. Trump’s election. The balance due is the agenda for Helsinki.