On Feb. 7, 1946, Arthur Terminiello, a Roman Catholic priest who was a fierce opponent of communism and believed that President Harry Truman was too comfortable with it, gave an incendiary speech in a Chicago hall that his sponsors had rented.

The hall held about 800 people, but nearly 2,400 showed up. Father Terminiello’s opponents outnumbered his supporters by a 2-1 ratio. The atmosphere in the hall was electric, with almost everyone present taking sides for or against this priest -- all under the watchful eyes of Chicago police.

The speech delighted the priest’s supporters and enraged his detractors. When it became apparent that violence might break out, the Chicago police approached Terminiello while he was speaking and asked him to stop and leave the building.

He refused to leave and resumed his speech. The police prediction soon came to pass. The fiery priest ignited the hatred of his adversaries, many of whom seemed to have come to that venue to silence him. The shouters hurled chairs, rushed the stage and attempted to attack him.

The police safely escorted Terminiello out of the hall and then, in the presence of the many rioters who by now had spilled out onto a public street, arrested him for inciting a riot. The charge was defined in Illinois in the mid-1940s so as to criminalize any behavior that intentionally arouses the public to anger or brings about public unrest.

The police did not arrest any of the rioters who smashed windows, destroyed the stage and assaulted the priest. They saw him arrested for his words that they hated.

Terminiello was tried and convicted. After his conviction had been upheld by the Illinois Supreme Court, he appealed to the U.S. Supreme Court, which reversed his conviction. In so doing, the high court saved the First Amendment from authoritarian impulses that sought to narrow its scope, and it ushered in the modern judicial understanding that has informed the present-day parameters of the freedom of speech.

The ruling generally barred the punishment of speakers who are expressing political opinions and held that the First Amendment needs breathing room; and breathing room contemplates that some people will hate what they hear and articulate that hatred.

The court warned the police against permitting audiences to silence speakers -- what lawyers and judges call “the heckler's veto.” Thus, the police today cannot throw up their hands and permit a speaker to be silenced as they did to Father Terminiello. They have an affirmative obligation to take all reasonable steps to protect the speaker’s right to speak, the audience’s right to hear and the protesters’ right to protest.

Fast-forward to last Saturday, also in Chicago, when Donald Trump canceled a rally and said he did so because he feared that protesters would disrupt it and some folks might be injured. Was this an example of the heckler's veto?

The legal issues here are complex and subtle, involving property rights and free speech. As a lessee of a government-owned building for his rally venue, Trump could not prevent any person from entering or remaining because of the person’s political views.

However, he could have asked the police to employ reasonable force to remove those whose behavior made it impossible for him to use the venue for the principal purpose for which he leased it. Since the First Amendment requires breathing room, the police must be extremely tolerant of protesters and may remove only those whose behavior physically prevents the use for which the venue was leased.

Stated differently, protest of political speech is itself protected speech, but protest cannot be so forceful or dominant that it vetoes the speaker.

What about the allegations that Trump himself is responsible for the violence at some of his rallies? If Trump publicly demands violence and there is no time or ability for any speech to neutralize his demands and the demanded violence takes place, his speech is unprotected -- and he can be prosecuted for incitement to riot. This is the modern rule that holds that all innocuous speech is absolutely protected, and all speech is innocuous when there is time for more speech to rebut or neutralize it.

When there is no time between the demand for violence and the responsive reactive violence, the speaker is liable for the violence he demanded. But if there is time for more speech to counsel against the violence, even if no neutralizing speech is actually uttered, the speaker cannot be prosecuted. And before any prosecution for speech may commence, the court must eliminate every possible lawful interpretation of the speaker’s words.

All these rules further the whole purpose of the First Amendment. It is to recognize, codify and protect the natural human right to form thought and to express the thoughts, and to encourage open, wide, robust, challenging speech about the government, uttered without a permission slip, free from government interference and without personal hesitation.

In the case of the canceled Trump rally last weekend, many fingers have been pointed. The Chicago police claim they never advised Trump to cancel. The Secret Service claims the same. Trump says he was the victim of ideologically driven fanatics who wanted to silence him, just as their predecessors did to Father Terminiello. If there is ever litigation over this, a jury will decide the facts.

But the law is clear. The First Amendment tolerates the maximum possible public discourse, disagreement and confrontations; and it commands the government to protect the values it embodies.

Andrew P. Napolitano, a former judge of the Superior Court of New Jersey, is the senior judicial analyst at Fox News Channel.