Rahman was bin Laden’s channel to the world. Their correspondence was the most important prize taken from bin Laden’s compound when he was killed May 2. They talked about everything: strategy, personnel, operations, political setbacks. Whatever thread still held al-Qaeda together passed from bin Laden through to Rahman.

The Libyan-born Rahman’s death blunts al-Qaeda’s ability to stage a new mega-attack against America; it brings the top leadership of the group closer to extinction; and it increases the likelihood that the organization’s center of gravity will shift from Pakistan’s tribal areas to one of the affiliates, such as the robust al-Qaeda in the Arabian Peninsula, based in Yemen.

Asked recently to name the most important remaining leader in al-Qaeda, a senior U.S. official had said it was Rahman. He explained that the nominal successor to bin Laden, Ayman al-Zawahiri, was actually a secondary figure — more a leader of the group’s Egyptian wing than of al-Qaeda as a whole. It would be in America’s interest if Zawahiri rather than Rahman were dominant, this official said, because Zawahiri was a divisive figure whose ad-hoc tactics were less threatening to America.

One of the subjects discussed frequently between Rahman and bin Laden was whether al-Qaeda’s ferociously violent tactics were alienating Muslims in the countries where it operated. That led to a fascinating 2005 missive from Rahman to Abu Musab al-Zarqawi, the head of al-Qaeda in Iraq, chiding him for targeting Shiite Muslims in his scorched-earth campaign in Iraq against America and its allies. And in more recent years, the two discussed the danger of seeking an Islamic “caliphate” in areas where al-Qaeda appeared strong, since that extremist move would likely alienate other Muslims. Better, they reasoned, to keep assaulting America.

Rahman’s death is especially important as the 10th anniversary of the Sept. 11, 2001, attacks on the United States approaches — and not just for symbolic reasons. Bin Laden had been working with Rahman to plan a spectacular strike against a U.S. target, pegged to the Sept. 11 anniversary. It’s not clear how far that planning had progressed, but whatever its level, it will be hampered, maybe even disrupted, by the death of the man whom bin Laden charged with organizing the details of the plot.

Also unclear is how the CIA was able to target Rahman in the Aug. 22 attack over North Waziristan or how he had been maintaining his sanctuary there. The cache of material taken from the bin Laden compound in Abbottabad in May didn’t include much that would help pinpoint the location of operatives in the field, according to the senior U.S. official, and Rahman would have understood that anything that disclosed his whereabouts had been compromised. But on targeting and other operational details, U.S. officials are tight-lipped.

Rahman fell to a Predator drone attack, the weapon that he and bin Laden had complained about so bitterly in their correspondence. Rahman had told his boss that this U.S. “intelligence war,” as bin Laden had called it, had made it nearly impossible for al-Qaeda to move, communicate, recruit or train in the tribal areas of Pakistan. They had discussed whether al-Qaeda should move its headquarters to someplace safer. That relocation seems more likely, now that the man who anchored the group’s presence in Pakistan is dead.