Encyclicals indicate high Papal priority for an issue at a given time. Only 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. Pontiffs differ on the use of encyclicals. On the issue of birth control and contraception, for example, Pope Pius XI issued the encyclical "Casti connubii", while Pope Pius XII spoke to midwives and the medical profession when he clarified his position 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, most of them after 1945, three of them ("Datis nuperrime", "Sertum laetitiae", and "Luctuosissimi eventus") protesting the Soviet invasion and crackdown of the Hungarian revolution in 1956. Pope Paul VI spoke about the war in Vietnam and Pope John Paul II, issued his protest against the war in Iraq in speeches. On social issues, Pope Leo XIII promulgated "Rerum novarum", followed by "Quadragesimo anno" by Pius XI, and "Centesimus annus" by 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. The magisterium of Pius XII is therefore significantly larger than the below listed 41 encyclicals. Most of the detailed teachings are in his papal speeches on specific topics such as: