Christopher Columbus, who I believe was saintly and should be canonized as a Saint, first discovered America and consecrated it to Our Lady. I believe he sailed in a vessel that was named after Her Immaculate Conception.
Anyway, here is Pope Leo: "We traverse in spirit and thought the wide expanse of ocean; and although We have at other times addressed you in writing - chiefly when We directed Encyclical Letters to the bishops of the Catholic world-yet have We now resolved to speak to you separately, trusting that We shall be, God willing, of some assistance to the Catholic cause amongst you. To this We apply Ourselves with the utmost zeal and care; because We highly esteem and love exceedingly the young and vigorous American nation, in which We plainly discern latent forces for the advancement alike of civilization and of Christianity.
2. Not long ago, when your whole nation, as was fitting, celebrated, with grateful recollection and every manifestation of joy, the completion of the fourth century since the discovery of America (1892), We, too, commemorated together with you that most auspicious event, sharing in your rejoicings with equal good-will. Nor were We on that occasion content with offering prayers at a distance for your welfare and greatness. It was Our wish to be in some manner present with you in your festivities. Hence We cheerfully sent one who should represent Our person. Not without good reason did We take part in your celebration. For when America was, as yet, but a new-born babe, uttering in its cradle its first feeble cries, the Church took it to her bosom and motherly embrace. Columbus, as We have elsewhere expressly shown, sought, as the primary fruit of his voyages and labors, to open a pathway for the Christian faith into new lands and new seas. Keeping this thought constantly in view, his first solicitude, wherever he disembarked, was to plant upon the shore the sacred emblem of the cross. Wherefore, like as the Ark of Noe, surmounting the overflowing waters, bore the seed of Israel together with the remnants of the human race, even thus did the barks launched by Columbus upon the ocean carry into regions beyond the seas as well the germs of mighty States as the principles of the Catholic religion."
https://www.vatican.va/content/leo-xiii/en/encyclicals/docuмents/hf_l-xiii_enc_06011895_longinqua.html
Finally, even if a nation was never Catholic, every effort must be expended toward making it Catholic Christian anyway. But America does in fact have Catholic roots, together of course with some Protestantism, a little Deism etc.
Lord Baltimore's Catholic colony in Maryland practiced Religious Tolerance:
http://www.catholicapologetics.info/apologetics/protestantism/protin.htm
"8. Catholic Maryland: The First Tolerant American Colony
A. Patrick O'Hare
"Catholics . . . were the first in America to proclaim and to practice civil and religious liberty . . . The colony established by Lord Baltimore in Maryland granted civil and religious liberty to all who professed different beliefs . . . At that very time the Puritans of New England and the Episcopalians of Virginia were busily engaged in persecuting their brother Protestants for consciences' sakes and the former were . . . hanging `witches'." (50:300-01)
B. Martin Marty (P)
"Baltimore . . . welcomed, among other English people, even the Catholic-hating Puritans (8) . . . In January of 1691 . . . the new regime brought hard times for Catholics as the Protestants closed their church, forbade them to teach in public . . . but . . . the little outpost of practical Catholic tolerance had left its mark of promise on the land." (9)
C. John Tracy Ellis
"For the first time in history . . . all churches would be tolerated, and . . . none would be the agent of the government . . . Catholics and Protestants side by side on terms of equality and toleration unknown in the mother country . . . The effort proved vain; for . . . the Puritan element . . . October, 1654, repealed the Act of Toleration and outlawed the Catholics . . . condemning ten of them to death, four of whom were executed . . . From . . . 1718 down to the outbreak of the Revolution, the Catholics of Maryland were cut off from all participation in public life, to say nothing of the enactments against their religious services and . . . schools for Catholic instruction . . . During the half-century the Catholics had governed Maryland they had not been guilty of a single act of religious oppression." (10)
D. Oxford Dictionary of the Christian Church (P)
"In the 17th century the most notable instances of practical toleration were the colonies of Maryland, founded by Lord Baltimore in 1632 for persecuted Catholics, which offered asylum also to Protestants, and of Rhode Island, founded by Roger Williams." (78:1383)
Stories of Protestant intolerance in America prior to 1789 could be multiplied indefinitely. Jefferson and Madison, in pushing for complete religious freedom, were reacting primarily to these inter-Protestant wars for dominance, not the squabbles of post-Reformation Europe. Here we are concerned with the immediate era of the Protestant Revolution - roughly 1517 to 1600, so the above anecdotes will have to suffice as altogether typical examples.
9. Conclusion (Will Durant)
"The principle which the Reformation had upheld in the youth of its rebellion - the right of private judgment - was as completely rejected by the Protestant leaders as by the Catholics . . . Toleration was now definitely less after the Reformation than before it." (122:456/11)