25% of the US military is Catholic

Prior to the creation of the Military Ordinariate and then the Archdiocese for the Military Services, the armed forces of the United States was served by an informal corps of volunteer priests. Beginning in 1917, the spiritual care of those in military service fell to the Military Ordinariate, the equivalent of a personal vicariate apostolic, that is, a particular church the membership of which is defined by some personal quality (as in this case being a member or a dependent of a member of the armed services) that is headed by a legate of the pope. Originally, the ordinariate was headed by then-Bishop Patrick J. Hayes, an auxiliary bishop of the Archdiocese of New York who served double duty as papal military vicar for the United States beginning on November 24, 1917.

Hayes was chosen because New York was the primary port of embarkation for U.S. troops leaving for Europe and therefore a convenient contact point for Catholic chaplains serving with them. When Cardinal John Farley, Archbishop of New York, died, Hayes was appointed as his successor and simply kept the additional title and duty of military vicar. In November 1939, the Holy See established the Military Vicariate of the United States of America. The post remained an additional duty of the archbishop of New York from Hayes’ time until Cardinal Terence Cooke began plans to separate it as its own jurisdiction in the early 1980s, plans he was unable to carry out before his death in 1983. His successor, Cardinal John Joseph O’Connor, a former Navy chaplain, former chief of Navy chaplains (the military’s title for its own senior chaplain officer) and former auxiliary bishop for the military, then assisted in creating the separate Archdiocese for the Military Services, USA, in 1985 and participated in the selection of its first own archbishop. On July 21, 1986, the military vicariate was elevated to the Military Ordinariate of the Archdiocese for the Military Services of the United States. As of April 2013, about 25% of the U.S. armed forces are Catholic.

Source