From The Conversation:
Nearly 200 years ago, in May 1846, Catholic priests and bishops named the Virgin Mary patroness of the United States of America – specifically, under her title as the Immaculate Conception, referring to the belief she was conceived without sin.
John Carroll, who became the first American bishop in 1790, was devoted to Mary throughout his life. In 1791, he and other American Catholic clergy consecrated the Diocese of Baltimore to Mary, asking her to “[preserve] from all evil” the people of the diocese. Half a century later, in 1846, a council of priests and bishops from across the country officially named Mary, under her title as the Immaculate Conception, the patroness of the entire United States, asking her for “the aid of her prayers.”