Tuesday night marked an extraordinary low point in this season of Middle East unrest. The attack on the US consulate in Benghazi killed four Americans, including Ambassador Christopher Stevens, a US foreign-service officer with a record of supporting the Libyan people during the rebellion against the Qaddafi regime.
Aside from the tragedy, and aside from the damage done to US-Libyan relations, this attack will have dangerous reverberations for the region. Once again, the actions of a few extremists have consequences that far outweigh their numbers or their popular support.
Although the details are still sketchy, the consulate attack had the hallmarks of a planned act of terrorism, not a spontaneous protest that turned violent. Well-armed militants fought off government troops sent to protect the consulate, a sad statement about the security failures in a country still run by militias. But Tripoli quickly responded by apologising and promising to bring the attackers to justice.
In Cairo, however, the Muslim Brotherhood called for nationwide protests against the Islamophobic film The Innocence of Muslims, which has served as a pretext for the attacks. There is every chance that this misguided anti-Muslim propaganda, the ostensible cause of the attack in Benghazi, will lead to further bloodshed.
To judge by its 14-minute trailer,this offensive filmis an amateurish affair and full of calculated insults against Islam. The filmmaker Sam Bacile, a California property developer who calls himself an Israeli Jew, has repeated vile insults against Islam at every opportunity; his shabby little film has been endorsed by the anti-Islamic Florida preacher Terry Jones, who stirred anger two years ago with threats to burn the Quran. It is an unfortunate fact of international affairs that such petty figures command any influence at all.
The film was also rumoured to be backed by US Jews and an Egyptian Copt - whether true or not, those reports raise frightening prospects of retaliatory attacks against blameless coreligionalists in Egypt and elsewhere. Governments across the Middle East must act firmly to protect minority groups from the extremist fringe that seeks further incitement. The dangerous fools in the United States have their counterparts in this region.
It is too early to weigh the full consequences for North African stability, for US relations with Libya and its neighbours, or for the growth of extremism. But certainly the timing - on the anniversary of September 11 and in the thick of US elections - will strike a chord in Washington's politics. Already the Republican candidate Mitt Romney has accused President Barack Obama's administration of sympathising with the attackers. In an already ugly campaign season, Mr Obama will be under extraordinary pressure to retaliate, first against the attackers in Benghazi. But given the inability of many US politicians to distinguish between the majority of Muslims and the extremist fringe, there will also be calls for a wildly irrational backlash.
The mobs in Cairo are responding to a childish, designed-to-offend film. They do exactly what the murderers in Benghazi - and cranks like Jones and Bacile - would want them to do.
Now it is Washington's responsibility to avoid overreacting to this provocation in Benghazi, despite the tragic deaths of Mr Stevens and his colleagues. Extremism on both sides fuels a feedback loop of outrage that benefits only the lunatic fringe. And who loses? All those who favour peaceful understanding between cultures.
Mature, prudent, thoughtful US candidates will respond to the killings with restraint. Now we'll see how many such politicians there are.