The June 3, 2016, email sent to Donald Trump Jr. could hardly have been more explicit: One of his father’s former Russian business partners had been contacted by a senior Russian government official and was offering to provide the Trump campaign with dirt on Hillary Clinton. The documents “would incriminate Hillary and her dealings with Russia and would be very useful to your father,” read the email, written by a trusted intermediary, who added, “This is obviously very high level and sensitive information but is part of Russia and its government’s support for Mr. Trump.” He replied within minutes: “If...