The cavernous room at the United Nations that usually houses sessions of that body’s Economic and Social Council quickly filled up with an after-hours audience of diplomats and scenesters come to see a self-appointed diplomat with no title but a penchant for both the headlines and frontlines.

Bernard-Henri Lévy — known as BHL —  had come to Turtle Bay to share the premiere of his new movie, “Why Ukraine,” which he co-directed with Marc Roussel. He shares writing credits with a longtime friend, Gilles Herzog. The movie is produced by Francois Margolin along with Emily Hamilton and Natalia Gryvniak. 

BHL was joined by the permanent representative of France to the UN, Nicolas de Rivière. It was clearly BHL’s show, in addition to his movie, as Mr. de Rivière limited himself to perfunctory remarks. There was a queue for BHL’s autograph and a standing ovation as the lights clicked back on after the credits.   

Mr. Riviere’s comments were circumspect, as this diplomat charged with representing Paris at the United Nations takes his marching orders from President Macron at the Élysée Palace. BHL, an intimate of French presidents but not their subordinate, is free to conduct his own freelance foreign policy, camera crew in tow.  

The peripatetic philosopher has long had his eye on President Putin’s hunger for a reconstituted Rusky Mir, or Russian world. This newest film features ample footage of BHL on the scene during the 2014 Maidan Revolution, swaddled in a scarf and speaking in French to cheering throngs. Ukrainian virtue and Russian avarice, BHL argues, have long been on view.  

We see footage of BHL meeting with a smooth-shaven Volodymyr Zelensky, then a candidate for president. BHL asks the former actor if he thinks he can make President Putin laugh; Mr. Zelensky quips that he could, but he wouldn’t want to. The exchange lands differently now, as does footage of Mr. Zelensky, just elected, yucking it up on camera. 

For BHL, the fight for Ukraine is a battle of light against darkness that he narrates in French with English subtitles in apocalyptic terms via omnipresent voiceover. At its best, “Why Ukraine” provides an arresting account of the war. BHL surveys the wreckage of Mariupol — “Putin’s apocalypse” — and the corpse-laden streets of small towns visited by cruelty.

BHL bivouacs with Ukrainian soldiers and shares their story, their uniform, and their rations. He video-conferences with a fighter hunkered down inside the Azovstal steel plant who expresses a readiness to die in service to the nation to which he has already given a limb. There has been no word from him since the bedraggled group surrendered.

One of the polemical thrusts of “Why Ukraine” is to defang the Russians’ professed goal of denazification. Mr. Lévy spends time with the group barricaded at Azovstal, the notorious Azov battalion that has long been dogged by accusations of leaning toward fascism. BHL gives it his kosher seal, noting its polyglot personality and Jewish members. 

BHL, who in the movie calls himself a “Jewish intellectual,” unfolds an argument that antisemitism no longer circulates in the land that hosted both the Cossack pogroms of the 17th century and is where one finds the corpse-filled ravines of Babi Yar. Footage of ecstatic pilgrims to the shrine at Uman of Rabbi Nachman adds to the argument that Ukraine is a cause for all.

“Why Ukraine” lands its most potent punches when it turns its attention to war’s surrealities. We see children advising that motor oil is a key component of Molotov cocktails and building mock checkpoints, as well as American veterans training Ukrainian troops in the arts of war and bellowing that a gun “is not a f—ing toy.” One survivor of the maelstrom has spent six weeks hiding in a shipping container.     

BHL promises that “Why Ukraine” will be followed by a second installment, “Slava Ukraini,” which will “document Ukraine’s victory.” For BHL, that victory means not only the expulsion of Russia from every inch of Ukraine, but also a war crimes tribunal reminiscent of Nuremberg. 

The philosopher hopes that Mr. Putin “spends the rest of his life in jail.” Should that come to pass, BHL, who claims that he has lived on “Ukraine time” for years, will surely be on hand for what would be the trial of the century. Only then, BHL notes wistfully, can he go back to teaching the work of Jean-Paul Sartre and William Faulkner.