In response to the attacks in Brussels on March 22, the U.S. State Department has issued an uncommonly wide-ranging travel alert, informing U.S. citizens to the "potential risks of travel to and throughout Europe."
These sort of blanket alerts are unusual, but not totally unprecedented. Just last November, the State Department issued a global travel alert in the wake of terrorist attacks in Beirut, Paris, and Mali. It should also be noted that the alert doesn't ban travel explicitly, but rather encourages greater vigilance while in Europe, particularly when in crowded public spaces or using mass transit, like metros and subways.
Two major things travelers can do right now: (1) Sign up for the State Department's Smart Traveler Enrollment Program (STEP), which issues live travel alerts and can help foreign authorities find you in case of an emergency and (2) figure out the local emergency number—911 isn't universal, after all.