Two centuries of Viking raids into Europe tapered off following the adoption of Christianity by King Olav TRYGGVASON in 994. Conversion of the Norwegian kingdom occurred over the next several decades. In 1397, Norway was absorbed into a union with Denmark that was to last for more than four centuries. In 1814, Norwegians resisted the cession of their country to Sweden and adopted a new constitution. Sweden then invaded Norway but agreed to let Norway keep its constitution in return for accepting the union under a Swedish king. Rising nationalism throughout the 19th century led to a 1905 referendum granting Norway independence. Norway remained neutral in World War I and proclaimed its neutrality at the outset of World War II. Nevertheless, it was not able to avoid a five-year occupation by Nazi Germany (1940-1945). In 1949, neutrality was abandoned and Norway became a member of NATO. Discovery of oil and gas in adjacent waters in the late 1960s boosted Norway's economic fortunes. The current focus is on containing spending on the extensive welfare system and planning for the time when petroleum reserves are depleted. In referenda held in 1972 and 1994, Norway rejected joining the EU.