This month’s nonfiction picks include a surprising look at a World War II veteran and a fresh dive into footage shot during the first year of Putin’s presidency.The proliferation of documentaries on streaming services makes it difficult to choose what to watch. Each month, we’ll choose three nonfiction films — classics, overlooked recent docs and more — that will reward your time.‘The Emperor’s Naked Army Marches On’ (1987)Stream it on the Criterion Channel.Whatever convinced the director Kazuo Hara that it would be wise to trail Kenzo Okuzaki, the subject of “The Emperor’s Naked Army Marches On,” it’s a rationale that probably shouldn’t be repeated, if it ever could be. Yet it resulted in one of the most jaw-dropping documentaries ever filmed. Screening as part of a collection of movies by Hara (whose wildly voyeuristic “Extreme Private Eros: Love Song 1974,” another excellent streaming choice, shows his ex-wife giving birth on camera), “Emperor’s Naked Army” has won praise from some of nonfiction filmmaking’s biggest names. Errol Morris put it on a list of his 10 favorite documentaries, saying: “I think it’s every interviewer’s dream that in the middle of an interview, when your subject is not forthcoming, you get up out of your chair and just beat them to a pulp. Of course, that never happens — except in ‘The Emperor’s Naked Army Marches On.’”The pugnacious interviewer — the man who physically pins men down during interrogations — is not Hara but Okuzaki. A World War II veteran, Okuzaki, at the time of filming, had spent more than a decade in prison for crimes that included murder and firing a slingshot at Emperor Hirohito. Now released, he is on a monomaniacal mission to learn more about the fates of some of his fellow Japanese soldiers who were killed in New Guinea after the war. The circumstances sound increasingly outlandish the more we hear, even as Okuzaki’s quest appears more unhinged (and at times darkly comic) in its single-mindedness. He even recruits people to role-play as relatives of the victims.With Hara tagging along as an observer and, by extension, perhaps an unwitting abettor, the reedy, loquacious Okuzaki, typically dressed in a suit, confronts potential witnesses and perpetrators and matter-of-factly demands that they talk, politely informing one that he came there prepared to beat him up if he does not. “When I committed a murder or when I shot at the emperor, I didn’t try to escape,” Okuzaki barks at another. “I took responsibility. But you didn’t. I hate irresponsible people.”“The Emperor’s Naked Army Marches On” is a journey alongside madness, an ethical quagmire and a uniquely volatile movie, one that has been difficult to stream stateside until now.‘Enemies of the State’ (2020)Stream it on Hulu.It’s difficult to describe this paranoia-suffused documentary directed by Sonia Kennebeck (and executive-produced by Errol Morris) without giving too much away. A second viewing is completely different from a first. “Enemies of the State” tries to untangle the case of Matt DeHart, an American who fled to Canada in 2013 and claimed that the F.B.I. had him physically tortured, ostensibly because he had stumbled on a bombshell revelation after spending time in hacktivist circles. His supporters were inclined to group him with Chelsea Manning and Edward Snowden, even though he never made his purported findings public. In the movie, only his mother, Leann, claims to have seen the files he found.But at the time DeHart fled to Canada, he had been indicted on charges of producing and transporting child pornography in the United States, in a case that he suggested had been concocted. And while some coverage of DeHart has noted the difficulties of verifying certain details — the story involves minors (on the one hand) and national security (on the other) — by the end of the film, Kennebeck has not only indicated what she thinks is true, but has also raised potent questions about confirmation bias. The movie suggests that the various agendas of DeHart’s supporters inclined them to view him in certain ways. Kennebeck prods viewers to question their own trustingness, pushing them to doubt certain interviewees, then to believe them and vice versa, and even to be skeptical of what they see. (Re-enactments synchronize original audio recordings with the lips of actors.)To say more would reveal too much, but “Enemies of the State” explains the saga with a clarity other accounts have lacked.‘Putin’s Witnesses’ (2018)Stream it on Ovid.Credit goes to the Museum of the Moving Image for introducing me to “Putin’s Witnesses,” which it screened earlier in the month. In this eerie documentary, the director, Vitaly Mansky, who was born in Lviv, Ukraine; studied film in Russia; and now lives in Latvia revisits footage he shot during the first year of Vladimir Putin’s presidency, beginning with Boris Yeltsin’s resignation on Dec. 31, 1999, a decision that elevated Putin to the position of acting president. In narration, Mansky says he started shooting the movie as P.R. for Putin’s campaign in the March 2000 election — although Putin portrays himself as being all-business, above doing the unsubstantive work of advertising or participating in a televised debate. At the same time, Mansky points out, he was always on TV. And part of what can be seen in “Putin’s Witnesses” is how people around him manufactured and softened his image. The director says he himself proposed that Putin pay a cuddly on-camera visit to an old schoolteacher in St. Petersburg.Yet Mansky sees things in the material that didn’t jump out at the time. He reflects on watching Putin with then-Prime Minister Tony Blair of Britain in the czar’s box at the Mariinsky Opera House: “It’s hard to picture the feelings of the guy raised in a St. Petersburg communal apartment, having joined the elite of the world at breakneck speed.” Mansky also spends time with Yeltsin and his family on election night and on the subsequent New Year’s Eve. Yeltsin looks increasingly perturbed at how much distance his chosen successor has put between them. Elsewhere, Mansky introduces various movers and shakers at Putin’s campaign headquarters on election night, then notes that the majority eventually either joined Putin’s opposition or were dismissed. (One of them, Anatoly Chubais, left his post as Putin’s climate envoy last week, reportedly over the war in Ukraine.)During his first year as president, Putin continues to act vaguely chummy with Mansky even as the faint rumblings of autocracy begin to be felt. Late in the movie, Putin praises the concept of being an elected leader instead of a monarch because it means a person like him can serve as president, then retreat into civilian life. “Everything you do with the state and the society today you will have to face in a few years as an ordinary citizen,” he tells Mansky. “It is a good thing to remember before taking a decision.” Those are chilling words now. More