For Reagan, a communist was a communist, fifty years and half a world of separation be damned. And the president was confident that he could spot a ringer. “Reagan’s personalized version of anticommunism also implied,” Mann writes, “that once a Soviet leader could establish that he was straightforward rather than deceitful and was trying to alter the Soviet system, then Reagan might be willing—more willing, in fact, than Richard Nixon—to give credence to that leader and to try to do business with him.” Thus was made the policy on which billions of lives rested.

Reagan trusted Gorbachev, despite the advice of nearly everyone around him. Trust, but verify. The origin of that durable phrase, Mann relates, is one Suzanne Massie, the most disquieting piece of the Reagan puzzle. Massie was a Russophile who fell in love with the motherland during a Russian language class. She went on to write a ridiculed book on the romanticism of Russian history, and also collaborated with her husband, Robert K. Massie, on the best-selling Nicholas and Alexandra. Mann dedicates a quarter of his book to the bizarre relationship between Massie and Reagan, highlighting how Massie passed along her superficial understanding of Russian culture to a credulous Reagan, who then recycled her tales during major summits. Massie, though, was right: Gorbachev was genuine about trying to open up the system. In listening to her, Reagan had once again done a fairly stupid thing that turned out to be correct. (His top aides, both threatened and bemused by Massie, eventually succeeded in cutting her off from the president.)

Mann devotes another quarter of his book to Reagan’s tear-down-this-wall utterance. He shows that Reagan made this demand against the wishes of his foreign-policy advisers, and largely for the purpose of appearing relevant back home, even as his presidency crumbled. But then again, discerning any motive at all behind Reagan’s actions is difficult, concedes Mann, noting that even Nancy never felt that she fully knew him.

Mann also dedicates a quarter of his book to Reagan’s relationship with Nixon, and another to his summits with Gorbachev. The four-part structure breaks up the narrative arc, as the reader sprints through the Reagan administration four times over. But the form does serve the purpose of building a solid argument, which will be tough to knock down.

Now, if Reagan was Gump in this drama, then Mikhail Gorbachev was Jenny: the smart, well-meaning lefty whose best efforts inevitably ended in disaster. (Forget the Forrest-Jenny romance; the analogy falls apart there.) Gorbachev’s strategy to open the Soviet system in order to strengthen it failed. And it would have failed, Mann argues, whether Reagan had hiked defense spending or not. The Soviet economy was in the tank, suffering from structural weaknesses that sapped the strength of the central government. Gorbachev’s only real ace in the hole was his cooperative relationship with Reagan.

Soviet conservatives, who battled Gorbachev’s reform efforts, lost their greatest talking point when the American president stopped acting like an enemy. Yet Reagan was able to strengthen Gorbachev just enough so that he could carry out his reforms, but not enough to keep him in power. And in the end, Reagan’s seemingly naïve view of the communist character turned out right. He always believed that if he could simply show Gorbachev the wealth the American system created, he’d be moved to change his mind. Gorbachev stuck to his ideological guns. But his opponents in the elite sphere of the Soviet government did not. They saw what riches lay before them for the taking. When Gorbachev, with Reagan’s help, opened the door, they walked in and took them. And then slammed the door shut again.

Ryan Grim is a congressional correspondent for the Huffington Post. He is a former staff reporter with and Washington City Paper and is the author of the forthcoming book, This Is Your Country On Drugs.