In the spring of 1166 Thomas was appointed Papal Legate for England, and he at once used his new authority to excommunicate in June all the king's chief agents - Richard of Ilchester, John of Oxford, Richard de Lucy, Jocelyn of Bailleul - while the king himself was only spared for the moment that he might have a little space for repentance. Rumour asserted too that the Primate acted as counsellor to the foreign enemies of England, declaring that he would either restore himself to his see or take away Henry's crown. He saw with delight the growing irritation of England under its sufferings after the Assize of Clarendon; ancient prophecies of Merlin's which foretold disaster were on his lips, and he grew yet more defiant in his sense of the king's impending ruin. The pride and temper of Henry kept pace with those of Thomas. He became more and more fierce and uncompromising. In answer to the excommunications he forced the Cistercians in 1166, by threats of vengeance in England, to expel Thomas from Pontigny. When papal legates arrived in 1167 with proposals for mediation, he bluntly expressed his hope that he might never see any more cardinals. His political activity was unceasing. He completed the conquest of Britanny, and concluded a treaty of marriage between his son Geoffrey and its heiress Constance. The Count of Blois was won at a cost of L500 a year. Mortain was bought from the Count of Boulogne. "Broad and deep ditches were made between France and Normandy." A frontier castle was raised at Beauvoir. His second son Richard, then twelve years old, was betrothed to Louis's daughter Adela; and his daughter Eleanor to the King of Castile. He secured the friendship of Flanders. He was busy building up a plan of Italian alliances and securing the passes over the Alps. Milan, Parma, Bologna, Cremona, the Marquis of Montferrat, the barons of Rome, all were won by his lavish pay. The alliance of Sicily was established by the betrothal of his daughter with its king. The states of the Pope were being gradually hemmed in between Henry's allies to north and south. The threat of an imperial alliance was added to hold his enemies in awe. In the spring of 1168 his eldest daughter was married to the Emperor's cousin, Henry the Lion, the national hero of Germany, second only to Barbarossa in power, Duke of Bavaria, Duke of Saxony, Lord of Brunswick, and of vast estates in Northern Germany, with claims to the inheritance of Tuscany and of the Lombard possessions of the House of Este. For the purpose of a judicious threat, he even entertained an imperial embassy which promised him armed help and urged him to recognize the anti-Pope, whose first act, as both Henry and Thomas well understood, would have been the deposition of the archbishop.
At last the moment seemed come, not only to win a peace with France, but to carry out a long-cherished scheme for the ordering of the Angevin Empire. He met the King of France at Montmirail on the feast of the Epiphany, January 6, 1169, and the mighty Angevin ruler bowed himself before his feebler suzerain lord to renew his homage. "On this day, my lord king, on which the three kings offered gifts to the King of kings, myself, my sons, and my land, I commend to your keeping." His continental estates were divided among his sons, to be held under his supreme authority. The eldest, Henry, who had in 1160 done homage to Louis for Normandy, now did homage for Anjou, Maine, and Britanny. Richard received Aquitaine, and Geoffrey was set over Britanny under his elder brother as overlord. This division of Henry's dominions by no means implied any intention on the king's part of giving up the administration of the provinces. It was but the first step towards the realization of his imperial system, by which he was to reign as supreme lord, surrounded by the sub-rulers of his various provinces. Harassed as he had been with ceaseless wars, from the Welsh mountains to the Pyrenees, he might well believe that such a system would best provide for the defence of his unwieldy states; "When he alone had the rule of his kingdom," as he said later, "he had let nothing go of his rights; and now, when many were joined in the government of his lands, it would be a shame that any part of them were lost." In the difficulties of internal administration the system might prove no less useful. That any serious difference of interest could arise between himself and the sons whom he loved "more than a father," Henry could never, then or afterwards, believe. He rather trusted that a wise division of authority between them might secure the administrative power in the royal house, and prevent the growth of excessive influence among his ministers. But for all his hopes, the treaty of Montmirail was in fact a crowning triumph for France; it was virtually the first breaking up of the Empire, and had in it the seeds of Henry's later ruin.