PRESIDENT BIDEN’S participation in the Group of Seven and NATO summit meetings this weekend was cast as a love fest, meant to underline the United States’ return to close relations with allies after four poisonous years of disharmony under President Donald Trump. The reaffirmation is important, of course; but it also means that the stiffest diplomatic challenges Mr. Biden will face in his first overseas trip as president will come in bilateral meetings outside the summits — in particular, with Turkish President Recep Tayyip Erdogan and Russian ruler Vladimir Putin.

Mr. Erdogan is the leader of a nominal U.S. ally and NATO member, while Mr. Putin has made himself into the most boldly aggressive U.S. adversary. But both are willful strongmen who were grossly mishandled by Mr. Trump, who admired their autocratic styles, excused or ignored their domestic repression and tolerated their assaults on vital U.S. interests. Though both regimes were punished by congressionally mandated sanctions, neither was deterred.

Mr. Biden’s most important task consequently will be to redraw red lines with two rulers who have worked against the United States in a host of ways. Mr. Erdogan, who will meet with Mr. Biden on Monday, has warred with U.S.-supported Kurdish forces in Syria and backed Azerbaijan’s recent offensive against Armenia. He has refused to give up his purchase of an advanced Russian anti-aircraft system even after that stance led to the cancellation of U.S. F-35 deliveries. His unrelenting repression of domestic dissent has included the prosecution of a U.S. academic under the absurd pretext that he directed an attempted coup against Mr. Erdogan.

Mr. Erdogan reportedly is offering Washington an olive branch in the form of a Turkish commitment to leave forces in Afghanistan to protect the international airport following the U.S. and NATO withdrawal. That would serve U.S. interests. But Mr. Biden should tell the Turkish leader that he will receive no waiver from existing sanctions without changes in his policies — and that future relations will depend on whether presidential and parliamentary elections due in 2023 are free and fair.

While the Biden-Erdogan session may be outwardly amicable, it will be no surprise if Mr. Biden’s first presidential meeting with Mr. Putin in Geneva on Wednesday produces fireworks. Mr. Biden is under pressure to strike a contrast with the toadyism of Mr. Trump, who openly sided with the Russian leader against U.S. intelligence agencies after a 2018 summit. The new president also has cause for stridency: Russian cyberattacks on U.S. targets have been unrelenting; Russian regulators are moving toward shutting down the operations of U.S.-sponsored Radio Free Europe; and mysterious attacks on U.S. diplomats are continuing for which Moscow is the leading suspect. Deterring Mr. Putin from further aggressions won’t be easy; Mr. Biden’s strongest card might be a promise to reveal and pursue Mr. Putin’s personal fortune, much of which is held outside Russia.

Administration officials have been talking about seeking a “more stable and predictable relationship” with Russia, and Mr. Biden would be right to pursue modest steps in that direction, such as agreeing to talks on nuclear arms issues and the restoration of some diplomatic links. But despite his assertions to the contrary, Mr. Putin probably does not want a more stable relationship with Washington. If he reins in his aggressions, it will be because he is effectively deterred by a president more willing to act forcefully in defense of the United States’ vital interests.

Read more: