Nationalism is the idea that all people who share a common language and a common culture should be organized in independent, self-governing states. Once upon a time, this posed a revolutionary threat to established boundaries. “Germany” and “Italy” were geographical expressions equivalent to saying “the Mid-West.” History had fractured each into multiple independent states. At the same time, the Russian Empire, the Austro-Hungarian Empire, and the Ottoman Empire were multi-lingual, multi-religious, and multi-cultural conglomerations. In many places, different “national” groups were mingled together. Beginning in the later 19th Century, Nationalism spread into all of these areas, leaving havoc and nation-states in its wake.
During the First World War, Britain and France agreed on how to partition the Ottoman Empire after victory had been won. After the war, the peacemakers in Paris tried to craft national boundaries that would gather as large a share of any national groups as possible into a coherent state. The best will in the world could not disentangle all of the groups, so national minorities grumbled in many parts of Europe.
Between the two world wars, predatory states fed on the grievances of national minorities, their own or those of others that created hostilities that could be exploited. So German minorities in Austria, Czechoslovakia, and Poland; the lands across the Adriatic that had been promised to Italy, but given to the Artist Formerly Known as Yugoslavia; Hungarians in Rumania; Poles in Czechoslovakia; Croatians and Slovenians in Yugoslavia; and all the lands lost by Russia in 1918. After the Second World War, the peacemakers drew the lines on maps, then shoved people where they wanted them. The problem of national minorities was solved.
The peacemakers also tried to freeze their lines in place for all coming time. In 1945 the newly-created United Nations outlawed “the threat or use of force against the territorial integrity or political independence of any state.” In essence, countries that existed had a right to continue existing in their original boundaries. The break-up of the Western colonial empires soon added many new nations to the world and to the rolls of those who accepted the United Nations’ prior decisions as their price of admission.
Now changed flows of power erode the established order. Vladimir Putin decided not to wait on plebiscites that the United States would never allow. He took back the Crimea and staged a limited invasion of two predominantly Russian “oblasts” of Ukraine. He has claimed that Belarus, Ukraine, and Russia “are one people.” Xi Jinping’s flouting of China’s agreement on the status of Hong Kong may be a preface to retaking Taiwan. In 2020, Chinese publications sent up trial balloons referring to parts of Tajikistan and Kazakhstan as once part of Imperial China’s domains. Water runs downhill, so lesser powers may soon start dusting off their claims.
Should this be stopped? Can this be stopped? Will this be stopped?
 The American Revolution can easily be portrayed as the first war of national liberation. The Dutch will object.
 The Sykes-Picot Agreement (1916) gave France Syria and Lebanon; while Britain got Iraq, Trans-Jordan, and Palestine. The British made a number of other commitments that did not accord well with reality, notably promising much of Turkey to Italy and Greece, an Arab state to the rulers of the Hejaz, and a “national home for the Jewish people” in Palestine.
 “Sub-Carpatho Ukraine, land that we love.” I stole that from Alan Furst, Kingdom of Shadows (2000).
 Quoted in Yaroslav Trofimov, “The Dangers in A New Era of Territorial Grabs,” WSJ, 19-20 September 2020.
 This amounted to a return to an established principle of 18th and early 19th Century diplomacy.