RUTHFULLY YOURS

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On Tuesday, after a four-day siege by terrorists who murdered at least 67 people, the Westgate shopping mall in Nairobi, Kenya, still appeared not to have been secured by government forces. The attack by more than a dozen heavily armed members of the Somalia-based al Shabaabterrorist group—including reportedly several Americans—was a thoroughly planned operation. Teams of the al Qaeda-affiliated terrorists entered the mall’s first and second floors in a two-pronged attack. The terrorists lined up hostages, separated Muslims from Christians and others, then tested the self-proclaimed Muslims on their knowledge of Arabic and Islam. Those who failed were shot, children included. The militants then settled in with the surviving hostages to repulse attempts to extract them.

Who are these terrorists? Al Shabaab—in Arabic, “the youth”—traces its roots to the militant wing of the Islamic Courts Union, an Islamist political movement that rose to power in Somalia in 2006. A U.S.-backed Ethiopian invasion of Somalia in December 2006 toppled the ICU, whose leaders fled. But al Shabaab remained and rapidly consolidated power as the dominant insurgent force. When less capable African Union peacekeepers replaced Ethiopian troops, al Shabaab recaptured much of southern and central Somalia by 2009.

There they might have remained if not for a United States- and United Nations-backed effort to beef up the African Union force. A far more robust and battle-tested Amisom (the African Union Mission in Somalia) pushed back al Shabaab, and by September 2012 the group had lost the strategic town of Kismayo, and with it much of al Shabaab’s revenue and reputation.

What followed is essential to understanding the resiliency of local Islamist extremist groups and the evolution of al Qaeda. As al Shabaab began losing ground in Somalia, a leadership battle ensued between those who wanted to be part of a global Islamist movement and others with more parochial aims. In 2012, al Shabaab declared itself part of al Qaeda, eliminating dissenters through assassination.

Al Shabaab’s decision to relegate the fight for Somali territory to secondary status was a major change. A group that had espoused more limited aims was suddenly abandoning its desire for local power in favor of a more idealized global fight. It is this model that is propelling al Qaeda, allowing the Islamist network to spread in Syria, Sinai, Iraq, Libya and Yemen.

Groups like al Qaeda in the Arabian Peninsula, Ansar al Shariah in Libya, Ansar Beit al Maqdis in the Sinai, the Islamic State of Iraq and Syria, and al Qaeda in the Islamic Maghreb have metastasized from local to networked global al Qaeda groups. Al Shabaab is following al Qaeda in the Arabian Peninsula, which first hit Saudi targets and then moved on to the United States.

Yet the U.S. government insists on calling al Shabaab a local, Somali threat, playing down the potential threat to the U.S. The Obama administration has clung instead to the narrative that al Qaeda is on the “path to defeat” and its remnants are “lethal, but less capable,” as President Obama put it in May.

A major tip-off in al Shabaab’s transformation was its effort to recruit Americans. According to U.S. officials, 50 or so U.S. citizens have made their way to Somalia since 2007 to join al Shabaab. This includes members of the Somali diaspora and non-Somalis, such as Syrian-American Omar Hammami, who called himself Abu Mansur al Amriki, aka “the American,” before he was killed earlier this month in Somalia, the victim of an internal dispute.

American Somalis began joining al Shabaab to fight the Ethiopians in 2007. Insight into its recruitment has come from the courtroom testimony of individuals who were arrested for providing material support to al Shabaab upon returning from Somalia: A local recruiter would promise vulnerable Muslim youths in the U.S. entry to paradise if they fought the Christian invaders. The youths would slip away for the trip to Somalia, often without the knowledge of their families.

The recruits receive military training in al Shabaab territory with other members of the Somali diaspora. A handful of Americans have worked their way up the ranks in al Shabaab, but many more were excluded from fighting the Ethiopians. Their lack of direct clan ties—to family members who could hold al Shabaab responsible for their deaths—made the Americans ideal candidates to become suicide bombers.

The first known Somali-American bomber, Shirwa Ahmed, participated in a suicide attack in northern Somalia in October 2008. Others followed, including Farah Mohamed Beledi and Abdisalan Hussain Ali, both from Minneapolis, who blew themselves up in separate attacks in 2011. Some young fighters became disillusioned upon seeing their compatriots siphoned off for cannon fodder. But there is no escape from al Shabaab, and many who attempted to leave have been executed.

A handful of Americans attempting to join al Shabaab have been stopped by law-enforcement while planning to travel to Somalia. Others, like Kamal Said Hassan and Abdifatah Yusuf Isse of the Minneapolis area, returned to the U.S. after training with al Shabaab. Convicted of providing material support to a terrorist group, Hassan was sentenced to 10 years in prison and Isse received a three-year sentence earlier this year.

U.S. intelligence officials fear that the score or so of American passport holders believed to be members of al Shabaab might return to the U.S. to commit terrorism. Al Shabaab’s leadership has not espoused attacks on America, but security experts fear that recruitment targeting Americans increases the probability of an attack.

Last month, al Shabaab released a video featuring what it called its “Minnesota Martyrs.” Minnesota is home to the largest U.S. population of Somalis. The 40-minute video, the first in a promised series, featured three Americans. The video glorified the three young men, saying they had given their lives on what is now a global battlefield. Although some within the group may see Africa as their battleground, those who have cemented the relationship with al Qaeda understand that jihad stretches from Morocco to the Philippines, from Tanzania to Iraq. And as al Qaeda leader Ayman Zawahiri has made clear, to the United States.

If reports now surfacing regarding Americans involved in the Nairobi attack—al Shabaab’s response to the Kenyan military presence in Somalia—are confirmed, it will be difficult for the Obama administration to continue claiming that al Shabaab is purely local. The terror group has the means for a major attack, and al Qaeda’s focus on the U.S. provides the motive.

From the triumphalism after Osama bin Laden‘s death to the president’s most recent speeches trumpeting an end to the war on terror, the Obama administration continues to proclaim al Qaeda’s demise. Implicit in this claim is that the Obama counterterrorism strategy is succeeding and “the tide of war is receding,” which in turn underpin substantial cuts to security spending and retreat from foreign entanglements. Alas, reciting a mantra does not make it true. Far from defeated, al Qaeda is stronger now than ever.

Ms. Zimmerman is a senior analyst at the American Enterprise Institute’s Critical Threats Project.

A version of this article appeared September 25, 2013, on page A17 in the U.S. edition of The Wall Street Journal, with the headline: Al Qaeda’s African Surge Threatens the U.S..