Contacts with the Holy See were first initiated by the Chaldeans during the Crusades. The earliest attempt at uniting with Rome was made by Sabrisho ibn al-Masihi (1226-1257), but it was Yaballaha III (1281-1317) who first made profession of the Catholic faith in a letter addressed to Pope Benedict XI, on 18 May 1304. His avowal, however, encountered heavy resistance from Nestorian bishops, and the union with Rome was of short duration. Such a union was attempted again, by Elias, the Nestorian bishop of Cyprus. The conversion of several bishops in 1445 resulted from the union with Rome made by Timothy, also bishop of Cyprus, which union lasted only five years. A rigid policy of Latinization was applied by the doge of Venice, who succeeded the Lusignans as ruler in 1489 and who had commercial undertakings in the Mediterranean Near East.
It was following the Council of Florence that the Nestorian patriarchs developed closer ties with Rome, owing largely to increased European influence among them. Their union in the sixteenth century was a consequence of the dissatisfaction of Nestorians with having members of the same family succeed to the patriarchate, especially when incompetent members occupied the position, like Simeon V Bar Mama's 8-year-old nephew. This dissatisfaction precipitated a meeting of the bishops of Erbil, Salamas, and Azerbaijan, joined by three or four delegates from the region, in which they agreed to select another patriarch; they chose Sulāqa, an especially religious priest who had served since 1540 as the superior of the convent of Rabban-Hormizd (some 40 kilometers from Mosul, in upper Iraq).
Sulāqa came to Rome on 15 November 1552, and, on the basis of a report submitted by Cardinal Maffei, Pope Julius III promulgated a bull on 20 February 1553 proclaiming Sulāqa the patriarch of Mosul. This date became the official birthday of the Chaldean Catholic church. Sulāqa was consecrated bishop by Pope Julius III in the Basilica of Saint Peter. To help Sulāqa, the pope appointed the Dominican Ambrose Butigeg as his representative to the Chaldeans of Mosul. Sulāqa returned with Khalaf, his companion on the journey to Rome, and took up residence for his patriarchate at Amida (present-day Diyarbakir) on 12 November 1553. Seven days later, his followers recognized him as head of the Chaldean "nation." He did not survive long, however. His archenemy, the Nestorian patriarch, Simeon Denha, lured him to Amida, where the Ottoman governor of the district arranged to have him drowned in the lake, on 12 January 1555. His successor, Abdeasho IV (1555-1565), was then recognized by the pope. Another successor, Elias VIII, sent a delegation to Rome with a recommendation from the Maronite patriarch, Makhluf of Lebanon, and a profession of faith. This delegation was escorted back to Iraq from Rome by two Jesuits.
Three centuries of conflict ensued between the main church and the branches that gravitated toward Rome, during which time a number of patriarchs were unable to sign professions of the Catholic faith and receive confirmation—largely because of their early deaths—until the time of Simeon IX (d. 1600). But the mainline Nestorian church resisted strongly this schismatic tendency, and heavy pressure compelled Simeon XIII to return to the Eastern church late in the seventeenth century. Diyarbakir, a Catholic center, remained without a "Catholic" patriarch until Yūsuf (Joseph) I, archibishop of Diyarbakir, was elected patriarch in 1672. Yūsuf took the advice of the Capuchin missionaries there and withdrew from communion with Mār Ilīya (Saint Elias) in 1672. He was recognized by Pope Clement X five years later as patriarch of the Chaldean "Catholics" and was designated in 1681 by Pope Innocent XI as Mār Yāsuf, but without a see. His position was reinforced after he obtained a firmân (imperial order) from the Ottoman government recognizing his autonomy from the Nestorian patriarch, Elias XII Denho. He traveled to Rome to receive confirmation from Pope Innocent XI, thus formalizing the union with Rome in 1680. The title of the head of the church was henceforth to be "patriarch of Babylon," although the occupant bore the name Mār Ilīya when he was residing at Ctesiphon.
Resistance from the Nestorian church continued to be strong, and the resultant pressures obliged Mär Yūsuf to resign and return to Rome, where he died in 1707. His successor, Mār Yūsuf II (1694-1713), strengthened the ties with Rome and gained the title "patriarch of Babylon," starting a line that continued until 1828, when Yūsuf V died. When Mär Ilīya VI was accepted as Catholic and was received into union with Rome, two Uniate "Nestorian" patriarchates came into being, at some time after 1692: the Mār Sham'un from the Sulāqa line, in Azerbaijan, and the Mār Yūsuf, in Diyarbakir.
The shift in titles and their redesignation created a great deal of confusion for scholars. The title Mār Ilīya signified descent from the old and venerated line of "Bayt al-Ab" (lit., "the House of the Father"). The title "patriarch" was hitherto accorded out of diplomacy to the Oriental Assyrians (another designation for Nestorians, because of their alleged ethnic descent from Assyrians) and in recognition of those who preceded in the Sulāqa line when they resided at Kotchannes. The title "patriarch of Babylon," moreover, was reserved for the Nestorian patriarchs of the Abūna-Basīdi family who resided at Rabban-Hormizd.
During all of these transactions and visits with Rome, the Maronites there played important roles in promoting the ties with the Holy See. Yūsuf (Joseph) al-Sim'āni (Assemani) and the Maronite patriarch served as middlemen in facilitating relationships between Rome and the breakaway Nestorians. They provided them with letters of recommendation to the Holy See and later printed their liturgical books.
Joseph III succeeded Joseph II, who had died of the plague in 1713. When Joseph III visited Mosul, he lured some three thousand Nestorians into the Catholic church, which, predictably, enraged the Nestorian patriarch. The Ottomans in the eighteenth century were under heavy pressures from the Catholic powers of Europe, who had generally surpassed them on the battlefields ever since the second siege of Vienna in 1683. The sultan's government relaxed its rules in favor of the Eastern-rite churches in Ottoman domains and made the concession to Rome of recognizing the Catholic branches at Diyarbakir and Mardin, leaving Mosul and Aleppo to the non-Uniate Nestorians.
The pressures on Joseph III caused him to journey to Rome and offer his resignation, but the pope rejected it. Meanwhile, war broke out between the Ottomans and the Persians, which kept him in Rome until 1741. He then returned to his flock and died in 1757. On 24 March 1759 the election of Joseph IV was recognized by the pope, who confirmed him as the patriarch of the United Chaldeans. He resigned in 1781 and was reinstated temporarily in 1791, then fully in 1793, as the patriarch administrator of Amid, where he died in 1796.
In 1802 the priest Augustine Hindi was named administrator of the patriarchate of Diyarbakir, and he was consecrated bishop of Mardin on 8 September 1804. He was then named apostolic delegate for the Chaldeans, a post he held for fifteen years. Rome had hoped, in denying him the patriarchate, to lure the Nestorian patriarchs of either Kotchannes or Rabban-Hormizd to create a single, united Chaldean patriarchate. Hindi did receive from Rome, however, the pallium, which carried with it the rank of archbishop. His death in 1827 ended the series of patriarchs of Diyarbakir, which had begun 147 years earlier.
John Hormizd, the last member of the Abūna family to represent the Chaldean Catholics, was appointed patriarch of Babylon in July of 1830 and so remained until his death on 16 August 1838. After once suspending him from the office, the Catholic church reversed itself and recognized Hormizd as patriarch of the Chaldeans, but only after he agreed not to admit any of his relatives to the episcopal order. In late 1844 he received an imperial firman from the Ottoman sultan, recognizing him as patriarch of the "Chaldeans" instead of the "Nestorians," as had been previously the case. With such recognition, the Chaldean church was now firmly established as an independent entity, free of Nestorian ties. It even received recognition as a millet (autonomous unit), separate from the Ottomans.
The lack of success that characterized the tenure of Coadjutor Nicholas Zaya resulted in his resignation and replacement in 1847 by Yūsuf Audo, who was confirmed the following year. Audo's long pontificate yielded many converts for, as well as great dissension with Rome, a dissension that began in 1860 with the question of jurisdiction over the Malabar Christian Uniates of southwestern India. A series of less heralded patriarchs succeeded Audo, who died in 1879. The only one to achieve distinction was Yūsuf Emmanuel II Thomas, whom the Holy See named apostolic delegate for the Nestorians on 9 July 1900. Yūsuf was a popular patriarch, who served in the Iraqi senate for twenty-five years after the state's formation. His long pontificate was marked by the return to Catholicism of several Nestorian villages, two bishops, and many members of the clergy. He also witnessed the massacres of 1918, when four bishops, many priests, and up to 70,000 of the faithful are said to have perished. Joseph VII Ghanima (1948-1957) succeeded him, followed by Paul II Cheikho in 1958.