The attack Friday at a Tunisian beach resort killed at least 18 Britons, but officials said Monday the actual number was likely to be 30. That would make it by far the deadliest terrorist attack on Britons since the London transport bombings 10 years ago. Yet it was just one of three terrorist attacks on three continents that followed one another in frightening succession last week. While it is not yet known whether the attacks were linked, they have left officials in numerous countries concerned about how to keep their citizens safe. Not long after that, a bomb ripped through...