Frederick IX was King of Denmark from 1947 to 1972. Born into the House of Glücksburg, Frederick was the elder son of King Christian X and Queen Alexandrine of Denmark. He became crown prince when his father succeeded as king in 1912. As a young man, he was educated at the Royal Danish Naval Academy. In 1935, he married Princess Ingrid of Sweden and they had three daughters, Margrethe, Benedikte and Anne-Marie. During Nazi Germany's occupation of Denmark, Frederick acted as regent on behalf of his father from 1942 until 1943. Frederick became king on his father's death in early 1947. During Frederick IX's reign Danish society changed rapidly, the welfare state was expanded and, as a consequence of the booming economy of the 1960s, women entered the labour market. The modernization brought new demands on the monarchy and Frederick's role as a constitutional monarch. Frederick IX died in 1972, and was succeeded by his eldest daughter, Queen Margrethe II.