Daughter of King Constantine I of Greece and his wife, Princess Sophia of Prussia, Princess Helen spent her childhood in Greece, Great Britain and Germany. The outbreak of World War I and the overthrow of her father by the Allies in 1917 permanently marked her and also separated her from her favorite brother, the young Alexander I of Greece. Exiled in Switzerland along with most members of the royal family, Helen then spent several months caring for her father, plagued by disease and depression. In 1920, the princess met Carol, Crown Prince of Romania, who quickly asked her hand in marriage. Despite the bad reputation of the prince, Helen accepted and moved to Romania, where she soon gave birth to their only son, Prince Michael, in 1921.