The beheading of U.S. freelance photographer James Foley, now confirmed by U.S. officials, recalls the similarly gruesome murder of Wall Street Journal reporter Daniel Pearl in 2002. It should act as a reminder, too, that the Islamic State began as al-Qaida-in-Iraq and differs from Pearl's killers only in tactics.

Both proclaimed killings of American journalists were acts of propaganda designed to shock. The leaders of Islamic State aren't naïve enough to believe that Foley's death will persuade the United States to end its airstrikes against the group. No more did al-Qaida's self-described 9/11 architect Khalid Sheikh Mohammed expect the U.S. would meet the demand, made on the video recording in which he beheaded Pearl, to release all prisoners from Guantanamo Bay.

It's especially chilling that Foley's purported executioner had a British accent - Pearl's kidnapper Ahmed Omar Saeed Sheikh was also a British national, radicalized while in Bosnia. At some point these people return home, unless killed or arrested. They will return brainwashed and brutalized.

Both al-Qaida and Islamic State are best understood as nihilist organizations that target anyone who doesn't share their inchoate ideology, rooted in a warped interpretation of fundamentalist Sunni Islam: their enemies range from moderate Sunnis, to Shiites, Christians, Kurds, minority religious groups such as the Yazidi - and, of course, the West.

As ever, success is the best recruiter. Because, until recently, Syrian President Bashar Assad and Islamic State left each other alone, focusing instead on taking territory from more moderate Sunni rebels in Syria, Islamic State thrived there. Now it has seized about a third of Iraq and, according to the Syrian Observatory for Human Rights, recruited 6,300 new fighters, about 1,300 of them foreign, in July alone. That raises their total to about 50,000 from 15,000 a year ago, the Observatory said.

That's a higher estimate than those of most Western analysts, but the trend is clear. Also important is that this membership boost occurred before the United States launched airstrikes, so it was not in response to Western involvement.

The Islamic State differs from al-Qaida in the priority it gives to building a Caliphate in the heart of the Middle East, rather than attacking the West. Yet this is a matter of priorities only: The ideology and methods are shared and when Islamic State fighters go home, it defies experience and logic to believe they won't continue the fight there - one returnee already attacked a synagogue in Belgium.

The question for President Barack Obama and other Western leaders isn't whether to fight the Islamic State, but when. Should it be now, when the group is not yet entrenched in Iraq, and when Assad - finally - has begun to move against it in Syria?

Or will it be once its fighters have established themselves in Iraq and Syria and move on to Jordan, as they moved on the Kurds? If they were to succeed in Jordan, why not the Gulf states, where they already recruit and solicit donations?

In Iraq and beyond, the nightmare prospect is of an al-Qaida that controls petro-states and enjoys the funds, space and leisure to train jihadists from around the world.

The apparent tragic death of James Foley should not be seen as a signal to step back in the belief that this will only be our fight if we make it so. Foley's killing just confirms what the Islamic State is and what it intends.

Marc Champion writes editorials on international affairs for Bloomberg View.