AYMAN AL-ZAWAHIRI, the second-in-command of Osama bin Laden, has delivered a video statement, broadcast by al Jazeera, threatening more terror by al Qaeda in Britain and in the United States. His rant must be taken extremely seriously.

First, while Osama bin Laden is a hateful, murderous Islamist radical, there is a difference in psychology between him and Zawahiri. Bin Laden falls into the category of those who became radicals to fill a void in their lives. The scion of a colossally rich family, bin Laden is said to have first arrived at the jihadist frontlines in Pakistan, decades ago, with the typical dress and manner of a cosmopolitan Saudi. His radicalism springs from resentment that the Saudi kingdom has strayed from the path of pure Wahhabism, the state religion.

Zawahiri is different. He was recruited to the Egyptian jihadist movement as a teenager, and his worldview has not changed since he became a young fanatic. In effect, he has never matured and never had a normal life, even though he was trained in medicine and has real standing as an Islamic scholar. Paradoxically, this contrasts with bin Laden, whose only professions have been those of playboy and terrorist.

But his Islamic studies give Zawahiri's statements more authority and credibility with young jihadist recruits, to whom the guidance of a Zawahiri surpasses that of a "media star" like Bin Laden.

Finally, the strain of Islamist extremism represented by Zawahiri, originating in Egypt with the Muslim Brotherhood or Ikhwan, has always been more vituperative in its view of the West than the straight Wahhabism of bin Laden. The Egyptians adopted the Wahhabi claims to represent the only genuine Sunni Muslims in the world, and their program for imposition of a strict Islamic state; but they also expressed a deeper hatred and contempt for Westerners based on their experience with British colonial rule. The same ideology marks the Pakistani Islamists off from the Saudis, who always accepted the need for alliances with the Christian powers in order to protect Wahhabi dominance in the Arabian peninsula (while they attacked Shia and other non-Wahhabi Muslims).

While it may seem counterintuitive, the new offensive of al Qaeda, represented by the two terror attacks in London and the Zawahiri declaration, may be expressions of weakness on the main front in Iraq. Iraqi Sunnis are turning away from Abu Musab al-Zarqawi and his murderous Wahhabi offensive. Shia and Kurdish authorities are consolidating. With the Iraq jihad in decline, al Qaeda needs to reassert its power by more bloodshed in Baghdad and new atrocities in London and other Western cities.

But when Zawahiri speaks, more than rhetoric is involved. Real dangers to public security cannot be denied. Now is the time for U.S. preemptive action at home, including monitoring of the ideological messages in Sunni mosques and stepped-up surveillance and arrest of known radical preachers on our soil.

Stephen Schwartz is a frequent contributor to The Weekly Standard.