Until about three years ago, Ukraine and Russia were thick as thieves. Their close friendship dated back centuries, to Ukraine's incorporation into the Russian empire in the 17th century. Throughout the years that followed, the two countries forged deep ties across every field imaginable, from agriculture and energy to infrastructure and defense. Even after Ukraine gained its independence in the wake of the Soviet Union's collapse, those links held strong. Russia remained Ukraine's biggest trade partner, and Kiev relied on Moscow for nearly all of its energy imports. It also counted on the Kremlin to buy its primary exports, including agricultural goods, steel and military equipment. But all of that changed when the Euromaidan uprising swept across Ukraine in February 2014. The overthrow of Ukraine's pro-Russia president, Viktor Yanukovich, and his replacement with an ally of the West upended Kiev's relationship with Moscow. It wasn't the first time Ukraine had given...