President Recep Tayyip Erdogan of Turkey came to Washington for his first meeting with President Trump with hopes of achieving a “turning point” in an alliance that is vitally important to both sides, but has been profoundly strained by policy issues involving efforts to defeat ISIS in Syria. His demands were mostly unreasonable, which means continued tensions.

The enduring image of Mr. Erdogan’s visit will not be the pomp at the White House but that of his security guards and other supporters beating up protesters outside the Turkish ambassador’s residence. The police described the attack as “brutal” and the State Department called it an intolerable assault on free speech. Eleven people were injured, including a police officer, and nine were taken to the hospital. Some Republican lawmakers said Turkey should apologize.

The Turkish Embassy blamed the demonstrators for “aggressively provoking Turkish-American citizens who had peacefully assembled to greet” Mr. Erdogan. Washington’s Metropolitan Police will now have to determine accountability. That could be tricky; Mr. Erdogan and his security people are back in Turkey. Mr. Trump, meanwhile, has shown a shocking indifference to Mr. Erdogan’s authoritarian crackdown on his own people, which in turn makes it even more important to show that the United States will follow the rule of law.

As for policy issues, Mr. Erdogan demanded, among other things, that the United States abandon the Syrian Kurds, whom he views as terrorists. But Mr. Trump had already approved a plan to arm Syrian Kurds so that they can help in the campaign to take Raqqa, the capital of ISIS’ self-declared caliphate. Mr. Erdogan considers the Syrian Kurds to be indistinguishable from the Kurdistan Workers’ Party or P.K.K., a separatist group fighting his government in Turkey. For their part, American military commanders have long argued that the Kurds are among the most skilled fighters in the region and must be part of the ground force if Raqqa is to be recaptured anytime soon.