On the sidelines of the Group of 20 summit in Germany last month, President Trump’s advisers discussed how to respond to a new revelation that Trump’s oldest son had met with a Russian lawyer during the 2016 campaign — a disclosure the advisers knew carried political and potentially legal peril. The strategy, the advisers agreed, should be for Donald Trump Jr. to release a statement to get ahead of the story. They wanted to be truthful, so their account couldn’t be repudiated later if the full details emerged. But within hours, at the president’s direction, the plan changed. Flying home...