As secretary of state, Hillary Clinton basked in a diplomatic “Moscow Spring,” seizing on Vladimir Putin’s break from the presidency to help seal a nuclear arms-control treaty and secure Russia’s acquiescence to a NATO-led military intervention in Libya. But when Putin returned to the top job, things changed. Clinton, the Democratic presidential nominee, has vowed to stand up to Putin if elected, drawing on her four years of ups and downs as the public face of President Barack Obama’s first-term “reset” with Russia. By comparison, her Republican opponent, Donald Trump, has rung alarm bells in Washington and Europe with his...