By George Friedman

The U.S. government issued a warning Oct. 3 advising Americans traveling to Europe to be "vigilant." U.S. intelligence apparently has acquired information indicating that al Qaeda is planning to carry out attacks in European cities similar to those carried out in Mumbai, India, in November 2008. In Mumbai, attackers armed with firearms, grenades and small, timed explosive devices targeted hotels frequented by Western tourists and other buildings in an attack that took three days to put down.

European security forces are far better trained and prepared than their Indian counterparts, and such an attack would be unlikely to last for hours, much less days, in a European country. Still, armed assaults conducted by suicide operatives could be expected to cause many casualties and certainly create a dramatic disruption to economic and social life.

The first question to ask about the Oct. 3 warning, which lacked specific and actionable intelligence, is how someone can be vigilant against such an attack. There are some specific steps that people can and should take to practice good situational awareness as well as some common-sense travel-security precautions. But if you find yourself sleeping in a hotel room as gunmen attack the building, rush to your floor and start entering rooms, a government warning simply to be vigilant would have very little meaning.

The world is awash in intelligence about terrorism. Most of it is meaningless speculation, a conversation intercepted between two Arabs about how they'd love to blow up London Bridge. The problem, of course, is how to distinguish between idle chatter and actual attack planning. There is no science involved in this, but there are obvious guidelines. Are the people known to be associated with radical Islamists? Do they have the intent and capability to conduct such an attack? Were any specific details mentioned in the conversation that can be vetted? Is there other intelligence to support the plot discussed in the conversation?