Encyclicals indicate high papal priority for an issue at a given time. Pontiffs define when, and under which circumstances, encyclicals should be issued. They may choose to issue an apostolic constitution, bull, encyclical, apostolic letter or give a papal speech. Popes have differed on the use of encyclicals: on the issue of birth control and contraception, Pope Pius XI issued the encyclical "Casti connubii", while Pope Pius XII gave a speech to midwives and the medical profession, clarifying the position of the church on the issue. Pope Paul VI published an encyclical "Humanae vitae" on the same topic. On matters of war and peace, Pope Pius XII issued ten encyclicals, mostly after 1945, three of them protesting the Soviet invasion of Hungary in order to crack down on the Hungarian Revolution in 1956: "Datis nuperrime", "Sertum laetitiae" and "Luctuosissimi eventus". Pope Paul VI spoke about the war in Vietnam and Pope John Paul II, issued a protest against the war in Iraq using the medium of speeches. On social issues, Pope Leo XIII promulgated "Rerum novarum" (1891), which was followed by "Quadragesimo anno" (1931) of Pius XI and "Centesimus annus" (1991) of John Paul II. Pius XII spoke on the same topic to a consistory of cardinals, in his Christmas messages and to numerous academic and professional associations.