Each new attack, like the London stabbings last week, brings a heightened sense of caution to an already wary population. Lindsey Liss made a sobering request of her 17-year-old daughter when she sent her on a trip to Europe last summer: Don’t wear a Star of David necklace charm. “There’s no reason to make yourself a target,” said Ms. Liss, a 51-year-old artist and moth... [1260 chars]