The violence by Muslims responding to comments by the pope fit a pattern that has been building and accelerating since 1989. Six times since then, Westerners have done or said something that triggered death threats and violence in the Muslim world. Looking at them in the aggregate offers useful insights.

• 1989 — Salman Rushdie’s novel “The Satanic Verses” prompts Ayatollah Khomeini to issue a death edict against him and his publishers, on the grounds that the book “is against Islam, the prophet, and the Koran.” Subsequent rioting leads to more than 20 deaths, mostly in India.

• 1997 — The U.S. Supreme Court refuses to remove a 1930s frieze that decorates the main court chamber and shows Muhammad as a lawgiver; the Council on American-Islamic Relations makes an issue of this, leading to riots and injuries in India.

• 2002 — The American evangelical leader Jerry Falwell calls Muhammad a “terrorist,” leading to church burnings and at least 10 deaths in India.

• 2005 — An incorrect story in Newsweek reports that American interrogators at Guantanamo Bay, “in an attempt to rattle suspects, flushed a Koran down a toilet.” The article is picked up by the Pakistani cricketer and politician Imran Khan and prompts protests around the Muslim world, leading to at least 15 deaths.

• February 2006 — The Danish newspaper Jyllands-Posten publishes 12 cartoons of Muhammad, spurring a Palestinian Arab imam in Copenhagen, Ahmed Abdel Rahman Abu Laban, to excite Muslim opinion against the Danish government. He succeeds so well that hundreds die, mostly in Nigeria.

• September 2006 — Pope Benedict XVI quotes a Byzantine emperor’s views that what is new in Islam is “evil and inhuman,” prompting the firebombing of churches and the murder of several Christians.

These six rounds show a near-doubling in frequency: eight years between the first and second rounds, then five, then three, one, and ½.

The first instance — Ayatollah Khomeini’s edict against Mr. Rushdie — came as a complete shock, for no one had hitherto imagined that a Muslim dictator could tell a British citizen living in London what he could or could not write about. Seventeen years later, calls for the execution of the pope (including one at the Westminster Cathedral in London) had acquired a too-familiar quality. The outrageous had become routine, almost predictable. As Muslim sensibilities grew more excited, Western ones became more phlegmatic.

Incidents started in Europe (Mr. Rushdie, Danish cartoons, Pope Benedict) have grown much larger than those based in America (Supreme Court, Rev. Falwell, Koran flushing), reflecting the greater efficacy of Islamist aggression against Europeans than against Americans.

Islamists ignore subtleties. Mr. Rushdie’s magical realism, the positive intent of the Supreme Court frieze, the falsehood of the Koran-flushing story (ever tried putting a book down the toilet?), the benign nature of the Danish cartoons, or the subtleties of the pope’s speech — none of these mattered.

What rouses Muslim crowds and what does not is somewhat unpredictable. “The Satanic Verses” was not nearly as offensive to Muslim sensibilities as a host of other writings, medieval, modern, and contemporary. Other American evangelists have said worse things than Rev. Falwell did about Muhammad. (The Southern preacher Jerry Vines called the Muslim prophet “a demon-possessed pedophile who had 12 wives” without violence ensuing).Why did the Norwegian preacher Runar Søgaard’s deeming Muhammad “a confused pedophile” remain a local dispute while the Danish cartoons went global?

One answer is that Islamists with international reach (Ayatollah Khomeini, CAIR, Mr. Khan, Imam Abu Laban) usually play a key role in transforming a general sense of displeasure into an operational fury. If no Islamist agitates, the issue stays relatively quiet.

The extent of the violence is even more unpredictable. One could not anticipate the cartoons causing the most fatalities and the pope’s remarks the fewest. And why so much violence in India?

These incidents also spotlight a total lack of reciprocity by Muslims. The Saudi government bans Bibles, crosses, and Stars of David, and Muslims routinely publish disgusting cartoons of Jews.

No conspiracy lies behind these six rounds of inflammation and aggression, but, examined in retrospect, they coalesce and form a single, prolonged campaign of intimidation, with more sure to come. The basic message — “You Westerners no longer have the privilege to say what you will about Islam, the prophet, and the Koran; Islamic law rules you, too” — will return again and again until Westerners submit or Muslims realize that their effort has failed.

Mr. Pipes (www.DanielPipes.org) is director of the Middle East Forum and author of “Miniatures” (Transaction Publishers).