President Obama called the shooting deaths of at least five Dallas police officers a “vicious, calculated and despicable attack” and a “wrenching reminder” of the sacrifices of law enforcement. Speaking from Warsaw on Friday after the first of a series of meetings with European Union and NATO leaders, the president said he had offered his condolences to the Dallas mayor overnight. “We are horrified over these events,” he said. “We stand united with the people in the police department in Dallas.” The officers were monitoring a peaceful protest Thursday of police shootings of black men this week in Minnesota and...