Pity the poor residents of Belle Plaine. They’re about to get a veterans memorial with satanic symbols in their public park — and it’s their own fault.

They allowed a Christian memorial earlier this year, opening the park to all memorials in order to avoid violating the constitutional prohibition against establishment of religion. Now they have to allow the satanic memorial as a matter of free speech. Whipsawed between two different clauses of the First Amendment, they probably don’t know what hit them.

To understand what’s happening in Belle Plaine and why it makes legal sense — if no other kind — you need to start with the complex, judge-made rules about what happens when religion and free speech interact.

Sometime last year, the Belle Plaine Veterans Club paid for and installed a new monument for Veterans Memorial Park in the town 45 miles southwest of Minneapolis. Close to a granite memorial inscribed with the names of the town’s war dead, the new memorial depicted in silhouette a soldier kneeling with a rifle in front of a cross that is roughly the size of a tombstone in a military cemetery.

The Freedom From Religion Foundation objected, arguing that the new memorial was an endorsement of Christianity in violation of the establishment clause of the First Amendment.

In my view, it wasn’t at all certain that the memorial was unconstitutional. Yes, it includes a religious symbol, and it’s certainly possible that a court would have found that, in context, it amounted to a preference for religion over nonreligion.

But it also would have been possible for a court to interpret the memorial as depicting a soldier kneeling before a grave that was marked by a cross, not before the cross in a religious sense. Anyone who’s been to military cemeteries like those in France or Italy knows how moving the fields of crosses can be.

Such a memorial might not register as an endorsement of religion, and would be constitutionally permitted. But Belle Plaine, which had a population of just 6,661 in the last census, didn’t want to go through costly litigation. So it removed the memorial.

That led to public objection, which persuaded the town to allow the memorial. That in turn led to a creative but high-risk legal solution.

According to U.S. Supreme Court doctrine, the establishment clause applies only when the government itself is acting.

But if private speakers, not the government, are talking, then there’s no problem with government endorsement of religion, because (in theory) no reasonable observer would think the government was expressing a view.

So Belle Plaine declared that it was creating a zone in the park in which any private actor could place a memorial. That’s known in free-speech law as a “limited public forum.”

In such a forum, the government opens its property to public speech. It can limit the subject matter, but it can’t limit the speakers’ viewpoints.

Thus, Belle Plaine could legally say that the designated area was being opened to veterans memorials. But it couldn’t say that only Christian or religious or secular memorials could be placed there. Under a landmark 1994 Supreme Court decision, Rosenberger vs. Rector and Visitors of the University of Virginia, religion counts as a viewpoint in a limited public forum.

The town’s purpose in creating the forum was, of course, to allow the silhouette memorial. But that permission came at a cost: The town would have to allow any veterans memorial, regardless of its viewpoint, in the same area.

On cue, the Satanic Temple of Salem, Mass., responded. Its proposed memorial is a square block of stone with satanic symbols like the pentagram carved on the sides and an upside-down military helmet perched on top. The temple claims that “this will be the first Satanic monument erected by Satanists on public property.”

The town can’t do anything to stop it — not anymore. Satanism is a religious viewpoint. It would violate Satanists’ free-speech rights to deny them access to the limited public forum. Most likely, the town couldn’t even require memorial sponsors to be town residents, because the forum is open for veterans-memorial speech generally.

Whether you think the result is absurd is a good test of whether you like this compromise carved out by the courts.

One old-fashioned secularist view would be that no one should be able to erect a religious monument, whether Christian or secularist, in the park. According to this logic, the fiction of making the park into a limited forum for public speech should be treated as absurd. This is government space.

An alternative old-fashioned critique, this one from the other side, would say that there’s nothing wrong with the town choosing a memorial that reflects its residents’ religious values. Endorsement of religion wasn’t the constitutional framers’ intent when they adopted the establishment clause. As long as no one is being coerced to worship, the framers would have objected only if government money was used to erect a religious statue. A privately funded memorial on public land wouldn’t have been considered a violation.

The current compromise tells government that it has to choose between control of monuments, in which case it can’t endorse religion, or giving up control and allowing anyone to speak.

Faced with these options, most towns choose control. Pleasant Grove, Utah, made that choice in a case that went to the Supreme Court. In 2009, the justices ruled that the town could exclude a proposed religious monument from a park because the choice of statues was a matter of government speech.

Belle Plaine probably now wishes it had gone that route. Or maybe Satan and the cross can coexist, a strange combination of bedfellows arranged courtesy of the justices in Washington.

Noah Feldman 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.”