Three soaring flagpoles stand on the plaza outside the entrance to Boston’s city hall. Two of the flagpoles are almost never disturbed — one always flies the American and POW-MIA flags, and the second displays the flag of Massachusetts. But what flies from the third flagpole varies. Often it’s the flag of Boston; often not. The city has a longstanding policy of allowing private groups to hold flag-raising ceremonies on the plaza, when they can replace Boston’s flag on the third pole with a flag of their choosing.

Such programs take place all the time, as anyone who lives or works near City Hall can attest. Private groups regularly gather on the plaza to celebrate the culture of another country and fly its flag from the third pole, frequently accompanied by music or speeches. Other groups celebrate not countries but causes, and have proudly flown their banners — including the rainbow Pride flag, the Juneteenth flag, the transgender rights flag, and the flag of the Bunker Hill Association — from the third pole.

“We commemorate flags from many countries and communities at Boston City Hall Plaza,” the city says on its website. “We want to create an environment in the City where everyone feels included.” It can be tough to get municipal approval for some things, but not for flag events. The city’s Property Management Department received 284 flag-raising applications between 2005 and 2017. It approved them all.

So when Camp Constitution — a religious volunteer association established to “enhance understanding of our Judeo-Christian moral heritage [and] our American heritage of courage and ingenuity” — requested permission to hold a one-hour program at City Hall Plaza to commemorate Constitution Day in 2017, it anticipated no objection. Its director, Harold Shurtleff of West Roxbury, filled out the simple application, noting that the group planned to raise the ecumenical Christian flag — which consists of a white field with a red cross in the upper left corner — and have Boston clergy deliver some short speeches focusing on Boston history.

But the city denied his request. When Shurtleff asked why, he was told that Boston had a policy of “respectfully refraining from flying non-secular flags” on City Hall flagpoles. In truth, the issue had simply never come up before. Only after turning Camp Constitution down did the city draft “flag raising rules,” the first of which is that the city will not display flags it deems “inappropriate or offensive in nature or those supporting discrimination, prejudice, or religious movements.”

Did City Hall have a right to say no to Camp Constitution? The answer to that question turns on a dispute over what the First Amendment mandates. A lawsuit to settle that dispute has been wending its way through the federal courts and is now before the First Circuit Court of Appeals.

Boston’s argument is that the three flagpoles are a government forum, that any flag they display amounts to government speech, and that no private party can compel the government to express a message it doesn’t choose to express. It relies on two important Supreme Court cases. In the 2009 case of Pleasant Grove v. Summum, the high court ruled that a town in Utah could reject an organization’s request to place a monument in a city park, despite the fact that another group’s monument had been accepted. In Walker v. Texas Division, the court held that license plates are tantamount to government speech, so Texas could refuse to produce a specialty plate honoring the Sons of Confederate Veterans.

By the same reasoning, Boston contends, it is allowed to restrict the flagpoles outside City Hall to flags the city wishes to be associated with — and it doesn’t wish to be associated with a message promoting a specific religion. Indeed, argues the city in its brief, it can’t do so without running afoul of the Constitution’s ban on “an establishment of religion.”

It’s a plausible argument, and to date the courts that have ruled in this case have accepted the city’s reasoning.

But Shurtleff and Camp Constitution make a counter-argument. City Hall Plaza and the third flagpole are public forums, they say. They point out that the city’s own printed guidelines for private groups requesting permission to hold flag-raising events call those venues “public forums” and say its policy is “to accommodate all applicants” — which is what Boston always did. For 12 years it accommodated every flag-raising request it received. Gregory Rooney, the Property Management Division commissioner, never asked to see a flag in advance. In fact, Rooney acknowledged, the Christian flag was problematic only because of its name. Had it been called “the Camp Constitution flag,” there would have been no objection.

The plaintiffs contend that the Summum case is irrelevant here. Monuments are installed in parks on the assumption that they will be permanent, as the Supreme Court emphasized in its opinion, while groups raising a flag outside City Hall occupy the third flagpole for merely an hour.

The Texas license plate case is inapposite as well, they argue. Like every state, Texas exercises direct control over the messages that appear on its license plates. License plates are state property, they are primarily used as a form of government ID, and they bear the state’s name. That makes them government speech.

City Hall’s third flagpole, by contrast, has traditionally been open to all flags, and the city never took a position on any message those flags conveyed. Can it really claim now that all the groups it said yes to over the years were actually engaging in speech sponsored and approved by the government? And only the Christian flag is beyond the pale?

From such interesting debates is First Amendment jurisprudence fashioned. In cases ranging from abortion clinic sidewalks to military funerals to St. Patrick’s Day parades, the Supreme Court has been ever more firmly restricting the government’s power to limit private expression. What might the nation’s highest court do if it ultimately decides the Boston flagpole case?

(Jeff Jacoby is a columnist for The Boston Globe).