Sen. John McCain says that GOP presidential front-runner Donald Trump doesn't understand the problems posed by Russia's new role in the Syrian conflict. "I don't think he understands very well the situation. And he's entitled to his opinion," the Arizona Republican said Sunday in an interview with CNN's Jake Tapper on "State of the Union." McCain was pushing back against Trump's assertion last week that the United States should let ISIS and Syria's army fight -- and let Russia worry about ISIS there. Trump's comment came as Russia launched air strikes in the region. "Do we want to keep slaughtering...