In bringing about the fall of the Turkish, Austrian, and Russian empires, World War I provoked significant changes in the structures of the Eastern Orthodox Church. On the western borders of what was then the Soviet Union, in the newly born republics of Finland, Estonia, Latvia, and Lithuania, the Orthodox minorities established themselves as autonomous churches. The first three joined the jurisdiction of Constantinople, and the Lithuanian diocese remained nominally under Moscow. In Poland, which then included several million Belorussians and Ukrainians, the ecumenical patriarch established an autocephalous church (1924) over the protests of Patriarch Tikhon. After World War II> the Estonian, Latvian, and Lithuanian autonomies were again suppressed, and in Poland the Orthodox church was first reintegrated to the jurisdiction of Moscow and later declared autocephalous again (1948).
In the Balkans changes were even more significant. The five groups of Serbian dioceses (Montenegro, the patriarchate of Karlovci, Dalmatia, Bosnia-Herzegovina, and Serbia) were united (1920–22) under one Serbian patriarch, residing in Belgrade, the capital of the new Yugoslavia. Similarly, the Romanian dioceses of Moldavia-Walachia, Transylvania, Bukovina, and Bessarabia formed the new patriarchate of Romania (1925), the largest autocephalous church in the Balkans. Finally, in 1937, after some tension and a temporary schism, the patriarchate of Constantinople recognized the autocephaly of the church of Albania.
The Soviets invaded Finland in 1939 and this initiated the start of the Winter War. Eventually Finland fell into to Soviet orbit, but the Soviets took heavy losses since the Finns were well armed with rifles.