Over the past two years, Yemen-based al-Qaida in the Arabian Peninsula's (AQAP) global operational and ideological reach have added significant strategic depth to the besieged al-Qaida Senior Leadership organization (AQSL) in Afghanistan and Pakistan. Now, with the leadership vacuum Osama bin Laden's death has created, coupled with an apparent succession crisis in Yemen, AQAP is likely, over the course of the next year, to displace AQSL as the "vanguard of the Muslim Ummah," as the group characterized itself in December, and become the principal driver of the al-Qaida movement's effort to attack the U.S. and its allies in Europe.

AQAP possesses the right combination of operational expertise, communication with its supporters, ideological influence and transnational reach to elevate it to this position. Reaching such a conclusion is by nature imprecise science, but many pieces of evidence support it, in both AQAP's actions and words.

Terrorists want to be associated with the winning team, and over the last 21 months AQAP has managed to do something AQSL has not accomplished essentially since September 2001: attempt major attacks against the U.S. homeland or significant U.S. or other interests worldwide. In August 2009, AQAP nearly assassinated Saudi Arabia's top counterterrorism official; on Christmas Day 2009, an AQAP operative smuggled explosives onto a U.S.-bound flight; and in October 2010, AQAP placed explosives onto two more U.S.-bound flights. Although these attacks did not cause casualties, they nevertheless reflect the lethal expansion of AQAP's skills and operational capabilities.

AQAP has also mastered social media to reach and recruit followers. Through sophisticated, bilingual outreach, targeting both Arabic- and English-speaking Muslim audiences, AQAP and its predecessor al-Qaida in Yemen have published slick online magazines, Sada al-Malahim in Arabic and Inspire in English. The latter taps the skills of web-savvy Pakistani-American Samir Khan, resulting in a "hip" publication designed for a younger, tech-capable audience. However, each serves several important purposes: in particular, spreading operational knowledge, such as instructions for simple explosives; sustaining the U.S.-against-the-Muslim-world narrative; offering "inspiring" stories of jihadists and "martyrs"; and delivering darkly mocking commentary on global affairs.

Perhaps as importantly, AQAP spreads its English-language message through spoken word, in the online lectures of Yemen-based, American-born preacher Anwar al-Aulaqi, who seems to serve as a sort of chief AQAP ideologue and external operations figure. It's through his vast library of YouTube videos that Mr. al-Aulaqi appears to have inspired attacks in the U.S. and U.K., including the Fort Hood, Texas, military base shootings, and the stabbing of a British parliamentarian. In contrast, AQSL labors to communicate on an increasing infrequent basis, with some recent attempts — including al-Qaida deputy leader Ayman al-Zawahiri's assessments of Middle East protests — appearing ever more disconnected from their audience. The details emerging about bin Laden's late courier and the videos of the isolated terrorist leader watching himself on television only seem to confirm this disconnect between AQSL and its followers. AQAP's statements arguably attract far more attention these days.

Moreover, just as AQSL built its mythology on its anti-Soviet jihad in Afghanistan, so has AQAP based its own narrative, in part, on its members' experiences in the crucible of detention at Guantanamo Bay, a key rhetorical tool and recruiting avenue for AQAP. For example, the October 2010 edition of Inspire featured articles about two members' experiences at the detention facility. AQAP deputy emir Said al-Shihri and religious figure Ibrahim al-Rubaish are former Guantanamo detainees, as was Muhammad al-Awfi, who surrendered to Saudi authorities after becoming an AQAP military commander. Several other AQAP members have brothers who were or remain in detention at Guantanamo. Mr. Al-Shihri and his colleagues have exploited their alleged mistreatment there to gain new recruits to AQAP.

Finally, there is no denying the relative freedom of movement AQAP enjoys, compared with AQSL. Yemen has for years been a permissive environment for al-Qaida. In fact, the first al-Qaida attack took place, almost unnoticed, in Yemen in 1992, against U.S. forces transiting Yemen for Somalia. In October 2000, al-Qaida attacked the USS Cole off the coast of Aden, Yemen. In February 2006, 23 al-Qaida members — including at least one involved in the Cole attack and AQAP leader al-Wahishi — escaped from a Yemeni prison.

The situation seems poised to improve for AQAP, given the sudden departure to Saudi Arabia of Yemen President Ali Abdullah Saleh after he was reportedly wounded. Conversely, according to media accounts over the past two years -- most recently this past weekend -- relentless missile strikes and other suspected U.S. counterterrorism operations in the Afghanistan-Pakistan region have decimated and isolated the upper ranks of AQSL. Moreover, Taliban leaders have signaled willingness to break with al-Qaida — thanks in part to disenchantment over the collateral effects of U.S. operations.

To be sure, AQAP's ascent to the pinnacle of the global al-Qaida conglomerate may only displace AQSL by a matter of shades, at least in the near term. Even with bin Laden's death, AQSL remains a lethal threat to U.S. interests. Moreover, absent a succession of major attacks on the U.S. homeland or other strategic interests, AQAP will strain to completely displace AQSL's long, rich and diverse mythology. Nevertheless, AQAP has become the kind of strategic threat in the global al-Qaida movement that will force the U.S. to confront what are likely to be unpalatable policy choices wherever it turns — especially if its key interlocutor in Yemen, President Saleh, has indeed lost power.

David Butterworth is a graduate student in Global Security Studies at the Zanvyl Krieger School of Arts and Sciences at Johns Hopkins University and works as a contractor at the Department of Defense. The views expressed here are his own. His email is butterworthdavid@comcast.net.