In the mid-2000s, the tiny Baltic nation of Latvia had a Janus-like quality. Scenic downtown Riga featured malls, restored medieval architecture, and Western hotels. Snatches of English conversation could be overheard from backpackers and British stag parties eager to explore a newly accessible (and relatively cheap) European capital. The 2004 accession of Latvia, Lithuania, and Estonia to the European Union seemed to herald a new era of openness and economic development for the Baltic states. The outskirts of the city, where I volunteered at a foster home to get out of a reckless driving charge in Virginia, were quite different. There were no baroque apartments or medieval guildhalls in the suburbs, only drab housing blocks. The most common second language was Russian, not because it had been widely adopted by the Latvians but because so many native speakers had relocated to the Baltics during the Soviet era. Since independence, large Russian minorities in Estonia and Latvia have been agitating for political autonomy and Russian language instruction in schools. Not coincidentally, the Baltics are a target for Russian cyberattacks, aggressive air patrols, and other provocations. Europeans often make self-congratulatory references to the continent’s post-1945 long peace. In truth, this settlement was built on the violent reorganization of the continent into ethnically coherent nation-states. Even today, countries and regions on the outskirts of Europe are plagued by the same internecine conflicts that wracked the entire continent in...