The State Dept.'s terror warning didn't specify any European countries or cities.

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.S. authorities are warning travelers to be careful in Europe, after intelligence officials uncovered evidence that al Qaeda may be planning a series of Mumbai-style attacks. State Department officials emphasized that they were not asking Americans to avoid travel to European countries — many of which have raised their domestic threat levels to "high" — but simply to take some precautions, such as registering with U.S. embassies and staying away from civil disturbances. With so few specifics, should they have bothered with the warning at all? (Watch an AP report about the terror warning)

Yes, the danger is very real: The United Kingdom alone has just uncovered several terrorist plots in "rapid succession," says Delia Lloyd at Politics Daily. And rising fears of Muslim immigrant groups are pushing tensions so high in Sweden, Germany, France, and Britain that violence could erupt even if terrorists don't strike. At this point, Europe is a "powder keg."
"Terrorist plots darken Europe's immigration debate"

The alert is too vague to help: This alert is "no help to travelers," says Ellen Creager at the Detroit Free Press. All the government is saying is that there is a "potential for terrorist acts on public transportation systems and other tourist infrastructure," without even mentioning which countries are most dangerous. That is too "vague" to do any good — "what are travelers supposed to do, report anyone who looks foreign?"
"Europe travel alert is little help to travelers"

Sometimes vague is better than nothing: This is a "tough" call for the Obama administration, says Mark Halperin at Time, especially with the holiday travel season fast approaching. Whenever the U.S. national security team detects unusually intense chatter in terrorist circles, it "must juggle the normal need to keep citizens informed and safe without inciting panic or hurting tourism." This alert probably isn't specific enough to get people to cancel travel plans — it is just a reminder that common sense helps keep you safe.
"Euro terror alert"