The expression “lone wolf” was once almost exclusively associated with white supremacists in the United States seeking to overthrow the U.S. government by violent means. Timothy McVeigh, the Oklahoma City bomber who killed 168 civilians in one of the most deadly terrorist acts in American history, was such a creature.

Since the Oklahoma City outrage, the lone wolf phenomenon has surfaced in a variety of countries, including Israel. In February 1994, the fanatical American-born Jewish settler Baruch Goldstein mowed down 29 Palestinians in Hebron, prompting Hamas to launch a systematic campaign of suicide bombings in Israeli cities.

In recent years, however, the term “lone wolf” has become virtually synonymous with Muslims, or Western converts to Islam, venting their anger, bitterness and resentment at the West.

Of late, lone wolves have struck in the United States, Canada, France, Britain and Belgium. And there is no telling where they will turn up next, leaving a trail of death, destruction, anxiety and fear in their wake.

Al Qaeda — which claimed responsibility for the obliteration of the World Trade Center in Manhattan on Sept. 11, 2001 — has for years been urging its followers to launch lone wolf attacks. Nidal Hassan, a U.S. army officer of Palestinian origin who killed 13 people in Fort Hood in 2009, may have been inspired by the call of Al Qaeda.

Al Qaeda’s spinoff organization, Islamic State, has adopted the same tactic. Since the United States began bombing Islamic State targets in Iraq and Syria, Islamic State has called on Western sympathizers to hit back with retaliatory strikes.

In Australia, an Iranian-born Muslim, Man Haron Monis, commandeered a cafe in central Sydney and held patrons hostage until commandos killed him. In Canada, Michael Zehaf-Bibeau, a convert to Islam with a criminal record, fatally shot a Canadian soldier standing guard at a war memorial in Ottawa before he himself was killed.

Meanwhile, lone wolf militants of Muslim descent have deliberately driven vehicles into pedestrians in three French towns: Dijon, Nantes and Joue-les-Toures. In one incident, the driver shouted jihadist slogans, in keeping with his ideological affinity with Islamic State.

It’s possible that the attackers in France were inspired by Palestinian lone wolves. In 2008, Alaa Abu Dhein, a resident of East Jerusalem, murdered eight students at a rabbinical seminary in western Jerusalem. More recently, Palestinians have driven tractors and cars into crowds of Israelis in Jerusalem. Palestinian lone wolves have also used guns and knives to attack Israelis. It would appear that the Palestinian assailants were not acting on orders from any particular organizations, though Hamas praised their courage.

The attacks in France, home to a large and growing Muslim community, have prompted the French prime minister, Manuel Valls, to warn that the “terrorist threat” is “undoubtedly the main challenge of our time.”

He has a point.

Most French Muslims are law-abiding citizens who wish to integrate into French society, as I discovered while on assignment in France some years back. But a small minority has lashed out, influenced by radical Islam, the incursions of Western colonialism into the Middle East in the past century and the Arab-Israeli conflict.

It’s a problem that may linger on for longer than we care to contemplate.

Some of the terrorists are clearly mentally unhinged. An Indiana State University study found that between 33 percent and 40 percent of lone wolves had identifiable mental health problems. Interestingly enough, French authorities in Dijon and Nantes announced  that the drivers who slammed into pedestrians were mentally unstable.

To no one’s surprise, one of the lone wolves, Mehdi Nemmouche, had fought for Islamic State in Syria before returning home. Nemmouche, it will be recalled, killed four Jewish tourists at the Jewish Museum in Brussels last May.

Observers believe that the internet has played a pivotal role in the radicalization of Muslims who become lone wolves. With just a click of the mouse, a person sitting in front of a computer can easily access jihadist sites, including those of Islamic State.

Increasing numbers of young Muslims have gone down that dangerous path, much to the distress of Westerners, and the end is not yet in sight.