In Israel they have to deal with this all the time and they use sirens. (Didn't the US plan for air-raid sirens during the cold war, too?) And when the Arab world attacked Israel on Yom Kippur they mobilized, so clearly there are some provisions. In fact, *halacha* is that when a life is in danger you can violate almost any other restrictions. If I'd needed to send a text message to alert a potential victim, for example, that'd be a no-brainer. Messages to let people know I was safe are fuzzier.

Wars are usually fought against nations or, at least, organizations (like Hamas, Al Quaeda, etc). This was an attack from an individual with like-minded friends, but I don't know that it qualifies as war yet. I don't know where those boundaries are.