Finland was a province and then a grand duchy under Sweden from the 12th to the 19th centuries and an autonomous grand duchy of Russia after 1809. It gained complete independence in 1917. During World War II, Finland successfully defended its independence through cooperation with Germany and resisted subsequent invasions by the Soviet Union, albeit with some loss of territory. During the next half-century, Finland transformed from a farm/forest economy to a diversified modern industrial economy; per-capita income is among the highest in Western Europe. A member of the EU since 1995, Finland was the only Nordic state to join the euro single currency at its initiation in January 1999. In the 21st century, the key features of Finland's modern welfare state are high-quality education, promotion of equality, and a national social welfare system, although the system is currently facing the challenges of an aging population and the fluctuations of an export-driven economy. Following Russia's invasion of Ukraine in 2022, Finland opted to join NATO; it became the organization's 31st member in April 2023.