The rest of the European Union nations are looking at the possibility of a British departure from the bloc with disbelief, trepidation and anguish. But they are also preparing to retaliate. If Britons do vote in a referendum on Thursday to leave the European Union, they can expect a tough and unforgiving response, with capitals across the Continent intent on deterring other countries from following the British example, European officials and analysts said. In other words, Britain will be made to suffer for its choice. With other issues pressing, including Greek debt, the migrant crisis and terrorism, the largest and...