When the TV show “Star Trek” first aired in the late 1960s, the program was nowhere near the blockbuster money machine of syndication and sequels it later became. Ratings were low. Only the sci-fi geeks cared.

But in the 1970s, fans watching reruns helped helped breathe new life into the franchise—in part, because they appreciated how the show took risks, sometimes wading into the most divisive issues of the day.

Like the war in Vietnam.

The show's creator, Gene Roddenberry, says that setting the drama in space gave him the distance to address hot-button cultural topics. “It seemed to me that perhaps if I wanted to talk about sex, religion, politics, make some comments against Vietnam, and so on..." he said, "that if I had similar situations involving these subjects happening on other planets to little green people—indeed it might get by. And it did.”

In early episodes, Roddenberry and the show’s other creators appeared to be more or less supporting America's interventionist role in the world, says cultural historian and author H. Bruce Franklin, history professor emeritus at Rutgers University and author of four books on the Vietnam war. Franklin also guest curated the '90s exhibit “Star Trek in the Sixties" at the Smithsonian's National Air and Space Museum.

On April 6, 1967, for example, producers aired “City on the Edge of Forever,” in which Enterprise captain James T. Kirk stops his medical officer Dr. Leonard “Bones” McCoy from saving the life of Edith, a prominent peace activist. His reason? Because if she lives, she will prevent the U.S. from getting into World War II in time to stop the Nazis. It’s an episode where Kirk goes back in time to try and correct the timeline—while also falling in love with the woman who needs to die to correct it.

The episode’s Vietnam War subtext came to the fore in the script-revision process, says Franklin. While the original script focused on the tragedy of doomed love, with no reference to Edith's peace activism, the revised script shifts the story focus. In it, first officer Spock speculates that if Edith were to live, she might spread her pacifist ideas, slowing America’s entry into World War II and thus altering its outcome.

In the episode as it aired in 1967, Spock’s speculation became a major plot point whose subtext was the growing anti-war movement of the time. Asked 25 years later whether show runners intended the episode to contain contemporaneous anti-Vietnam-war references, producer Robert Justman replied, "Of course we did."

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In “A Private Little War” (aired Feb. 2, 1968), the Enterprise crew discovers that their Klingon enemies have been arming one tribe on a primitive planet with flintlock muskets. After Kirk gives muskets to the other tribe, claiming it will create a balance of power, doctor McCoy strenuously objects. This excerpt from an episode transcript echoes the Cold War superpower tensions that led to America’s containment policy—and ultimate involvement—in Southeast Asia. Kirk even makes a direct reference to the Vietnam War:

MCCOY: I don't have a solution! But furnishing them firearms is certainly not the answer!

KIRK: Bones, do you remember the 20th-century brush wars on the Asian continent? Two giant powers involved, much like the Klingons and ourselves. Neither side felt they could pull out.

MCCOY: Yes, I remember. It went on bloody year after bloody year.

KIRK: What would you have suggested—that one side arm its friends with an overpowering weapon? Mankind would never have lived to travel space if they had. No. The only solution is what happened back then: balance of power.

“It’s what the U.S. was trying to do in Vietnam,” says Franklin, referring to the American efforts to limit Soviet expansion and deter a nuclear showdown between Cold War superpowers.

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By early 1968, American public opinion about the war underwent a significant shift.

In February of that year, North Vietnam shocked the U.S. with the Tet Offensive, a massive surprise attack on American and South Vietnamese strongholds. A month later, American soldiers committed atrocities against Vietnamese civilians in the My Lai massacre. The takeaways were tough: The war was increasingly unwinnable. The U.S. government had been lying about that fact as it sent more young men to fight. And Yankees weren't always the good guys.

Around the same time, the show creators seemed to undergo their own radical shift. Case in point: “The Omega Glory,” episode 23 in the series’ second season, which is blatantly anti-war. To make his point, Roddenberry puts the Enterprise crew on a planet with two bitterly warring tribes, the Yangs and Kohms, with subtexts about biological warfare and the immorality of outside interference. If those names weren't obvious enough, the Yangs (Yanks) have somehow in their history obtained an exact copy of the original U.S. Constitution, and revere it as a sacred text—though they don’t understand it.

In the climactic scene, Kirk holds up the Constitution before the chief of the victorious warring faction, declaring that the document and its principles of basic human rights were written for all people, even one's enemies.

But while Kirk was touting America's ideological superiority, Franklin says, declaring that Communists (or Kohms) deserved the Constitution’s protections was a dangerous risk to take on television at that moment in history.

More than a decade after U.S. Senator Joseph McCarthy convened 1954 Senate hearings to identify and condemn anyone believed to have Communist sympathies, tens of millions of patriotic Americans still regarded Communists not only as enemies, but as toxic carriers of an ideological disease: "red fever." And even though mass anti-war protests had broken out around the country by 1968—questioning why young U.S. men were being sent across the world to fight and die to stave off Communism—there were still plenty who thought those protesters disgraced the most heroic, generous and decent nation on the planet.

The episode aired just days after the Tet Offensive ended, leaving nearly 4,000 American soldiers dead in only a month of fighting. Roddenberry’s message was timely.

“The Omega Glory” could have ruined Roddenberry, who was already pushing the show upstream against terrible ratings and pressure from NBC executives. By 1968, “Star Trek” was losing $15,000 an episode, the equivalent of $500,000 per episode today, says Marc Cushman, author of These Are the Voyages, a history of the show.

“Later on, when it became hugely successful, ‘Star Trek’ became an enormous industry, with a whole different set of values than what they had in the beginning,” says Franklin. “But in the beginning, they tried to say something.”