For Europeans, the Madrid bombings in March served as a startling wake-up call, as did a recent truce-or-consequences pledge from Osama bin Laden.
But wake-up calls to do what, exactly?
In the weeks since Spain's "3/11," British, Belgian, French and Spanish authorities have conducted extensive sweeps for terrorist suspects. The European Union met and appointed a terrorism "czar." It also agreed to coordinate intelligence more closely among member countries.
These steps point to a new focus on antiterrorism measures. But Europeans would be making a mistake if they limit their efforts to counterterrorism alone.
For more than a decade, another alarm has been sounding, not as shrill, but more important. That's the alarm of major demographic change - a dramatically aging and declining European population alongside the migration of millions of Muslims from North Africa and the Middle East.
In the larger sense, the migration itself is not the threat. After all, Europe has needed these workers to support a welfare state, and many of them are now citizens of their host countries. The danger is in how Europe reacts to this population influx.
Historically, Muslims have not succeeded in spreading far into Europe. Spain expelled the Moors in 1492. The Ottoman advance was stopped at Vienna in 1683.
In contrast, today's wave, sparked in the 1960s by the lure of economic growth on the Continent, is spread out and staying. Rotterdam is soon to be the site of Europe's largest mosque. France is home to 4.5 million Muslims, the biggest Islamic population in Europe. Muslim Turkey persistently knocks for entrance on the EU's door.
Europe's war on terrorism can't be separated from its Muslim community at large. Leaders will need the cooperation of the greater Islamic population to ferret out terrorists. Last month, Islamic leaders in Britain showed how this cooperation could work. They sent a letter to Muslim scholars, imams, and others, asking that it be read aloud in the country's 1,000 mosques. The letter asked followers to shun extremism and violence.
This example illustrates that Europe needs to win the hearts and minds of its Muslims, but it must not lose sight of the larger issue of its own destiny in this confluence of cultures.
How this coming together will turn out, no one knows. Will an anti-Muslim backlash prevail, or the opposite, Islamicization? Or will the communities simply coexist, or blend?
Jihad in Europe can prompt a reexamination of the Christian-Muslim relationship for the defined purpose of public safety. Or, preferably, it can move the Continent further down the road to a redefinition of its identity.