Several eastern Baltic tribes merged in medieval times to form the ethnic core of the Latvian people (ca. 8th-12th centuries A.D.). The region subsequently came under the control of Germans, Poles, Swedes, and finally Russians. A Latvian republic emerged following World War I, but the USSR annexed it in 1940 -- an action never recognized by the US and many other countries. Latvia reestablished its independence in 1991 after the breakup of the Soviet Union. Although the last Russian troops left in 1994, the status of the Russian minority (some 25% of the population) remains of concern to Moscow. Latvia joined both NATO and the EU in 2004; it joined the euro zone in 2014 and the OECD in 2016.