Cities worldwide are gearing up for New Year celebrations, from fireworks in Rio and Sydney to cultural traditions in Asia.

Russia and other hostile states have become increasingly brazen in adopting “gray zone” attacks against Europe and the United States, leaving defense officials with a dilemma: How to respond?

A man who pledged allegiance to the terrorist group carried out a deadly attack in New Orleans on New Year’s Day. By Alissa J. Rubin Liri Albag, 19, is one of about 100 hostages believed to ...