During Turkish president Recep Tayyip Erdogan’s visit to Washington, his “bodyguards” viciously beat and kicked Kurdish protesters outside the Turkish Embassy. It happened on a busy news day, to say the least, around the time of the revelation that President Donald Trump had asked the FBI director to stop investigating a former national security adviser’s ties to Russia. But this shameful episode shouldn’t be allowed to escape analysis and serious follow-up. Federal law enforcement must investigate and criminally charge the bodyguards — who should not be allowed to hide behind diplomatic immunity.


Here’s why the attack is so significant: It fundamentally erodes the First Amendment value of peaceful protest, right in the nation’s capital. In the U.S., the protesters have every right to speak out on behalf of Kurdish rights — or any other topic they want to trumpet.


In Turkey, free-speech rights for Kurds have always been very weak. For years, the Kurdish language itself was banned. As part of the Turkish effort to assimilate Kurds while destroying their identity, Kurds were referred to officially as “mountain Turks.” Peaceful advocates for Kurdish rights have long been accused of terrorism, even when they had nothing to do with the PKK, the Kurdish group that in some historical periods did use terrorism to assert its claim to autonomy or independence.


Erdogan’s men acted just as they would have in Turkey, where Kurds have little or no legal protection from government forces.


The other important piece of context is the day-by-day erosion of free speech in Turkey under Erdogan. Not only Kurds, but also all other opposition voices, are being curtailed. Thousands of journalists, lawyers, judges and others have been jailed since the attempted coup against Erdogan last year.


The actions of Erdogan’s men would need to be addressed under any circumstances. But it’s especially important now because the U.S. is at its own important juncture with respect to violence and free speech. For example, on university campuses, violence has silenced some speakers and the threat of violence has led to canceled events like Ann Coulter’s at Berkeley. If violence is allowed to limit or block speech, then the First Amendment isn’t working.


And let’s not forget — as Erdogan surely has not — that Trump’s rallies when he ran for president were marred by the violent removal protesters on several occasions.


There’s even a lawsuit against Trump and his campaign claiming that Trump incited the violence. Trump’s lawyers have claimed that the candidate’s own free speech rights would be violated by a verdict against him. They’ve also claimed that the campaign had the right to exclude the protesters, possibly by force.


The Turkish attack sends the message that Erdogan believes Trump is just like him, and that the U.S. is therefore just like Turkey when it comes to suppressing dissent. That’s the message that needs to be refuted, and fast.


That requires a full investigation. D.C. police on the spot were outnumbered and managed to arrest just two people, although nine were reported injured and a police officer was assaulted. We don’t know whether those arrested were the Turkish assailants or not.


That’s woefully inadequate. On video, several of the bodyguards, definitely more than two, can be seen committing criminal assault.


In general, D.C. police are loath to get involved in prosecuting foreign security personnel who are working at foreign embassies. The diplomatic consequences are too tricky.


For that reason, this investigation needs be taken over by federal authorities. The perpetrators have to be identified from the video and criminally charged.


Even if the guards have now been repatriated to Turkey, there is symbolic value in indicting them on assault charges. At minimum, they would not be able to return to the U.S.


As a matter of law, diplomatic immunity does not extend to the intentional use of violence to suppress First Amendment-protected free speech. That’s intervention in U.S. domestic affairs. Criminal charges are needed to vindicate the right of peaceful protest.


Will an investigation cause a diplomatic incident between the U.S. and Turkey, which remains a key NATO ally despite recent tensions? If the answer is yes, the blame should rest with the Turkish side, not the U.S. The Turks were the ones running amok. That has consequences — or at least it would in a rule-of-law democracy.


If federal law enforcement does not announce an investigation, that will also be read as a sign, domestically and globally. It will signal that the U.S. either doesn’t have the courage to defend its principles, or else that the Trump administration embraces Erdogan’s tactics and anti-Kurdish policies.


Inaction would itself be a diplomatic incident, with consequences far more serious than investigation and prosecution.


Noah Feldman is a Bloomberg View columnist. He is a professor of constitutional and international law at Harvard University and was a clerk to U.S. Supreme Court Justice David Souter. His seven books include “The Three Lives of James Madison: Genius, Partisan, President” and “Cool War: The Future of Global Competition.”