The next year saw him drawn across the Channel, where he was already master of a third of the present France. He had inherited Anjou, Maine, and Touraine from his father, Normandy from his mother, and the seven provinces of the South, Poitou, Saintonge, the Angoumois, La Marche, the Limousin, Perigord, and Gascony belonged to his wife. As Duchess of Aquitaine Eleanor had claims on Toulouse, and these Henry prepared in 1159 to enforce by arms. He was however luckless in the war. King Lewis of France threw himself into Toulouse. Conscious of the ill-compacted nature of his wide dominions, Henry shrank from an open contest with his suzerain; he withdrew his forces, and the quarrel ended in 1160 by a formal alliance and the betrothal of his eldest son to the daughter of Lewis. Thomas had fought bravely throughout the campaign, at the head of the 700 knights who formed his household. But the King had other work for him than war. On Theobald's death he at once forced on the monks of Canterbury, and on Thomas himself, his election as Archbishop. His purpose in this appointment was soon revealed. Henry proposed to the bishops that a clerk convicted of a crime should be deprived of his orders, and handed over to the King's tribunals.

The local courts of the feudal baronage had been roughly shorn of their power by the judicial reforms of Henry the First; and the Church courts, as the Conqueror had created them, with their exclusive right of justice over the clerical order, in other words over the whole body of educated men throughout the realm, formed the one great exception to the system which was concentrating all jurisdiction in the hands of the king. The bishops yielded, but opposition came from the very prelate whom Henry had created to enforce his will. From the moment of his appointment Thomas had flung himself with the whole energy of his nature into the part he had to play. At the first intimation of Henry's purpose he had pointed with a laugh to his gay attire - " You are choosing a fine dress to figure at the head of your Canterbury monks; " but once monk and primate, he passed with a fevered earnestness from luxury to asceticism. Even as minister had opposed the King's designs, and foretold their future opposition: " You will soon hate me as much as you love me now," he said, "for you assume an authority in the affairs of the Church to which I shall never assent." A prudent man might have doubted the wisdom of destroying the only shelter which protected piety or learning against a despot like the Red King, and in the mind of Thomas the ecclesiastical immunities were parts of the sacred heritage of the Church. He stood without support: the Pope advised concession, the bishops forsook him, and Thomas bent at last to agree to the Constitutions drawn up at the Council of Clarendon. The King had appealed to the ancient " customs " of the realm, and it was to state these "customs" that a court was held at Clarendon near Salisbury. The report presented by bishops and barons formed the " Constitutions of Clarendon," a code which in the bulk of its provisions simply re-enacted the system of the Conqueror. Every election of bishop or abbot was to take place before royal officers, in the King's chapel, and with the King's assent.

The prelate elect was bound to do homage to the King for his lands before consecration, and to hold his lands as a barony from the king, subject to all feudal burthens of taxation and attendance in the King's court. No bishop might leave the realm without the royal permission. No tenant in chief or royal servant might be excommunicated, or their land placed under interdict, but by the King's assent. What was new was the legislation respecting ecclesiastical jurisdiction. The King's court was to decide whether a suit between clerk and layman, whose nature was disputed, belonged to the Church courts or The King's,, A royal officer was to be present at all ecclesiastical proceedings, in order to confine the Bishop's court within its own due limits, and a clerk once convicted there passed at once under the civil jurisdiction. An appeal was left from the Archbishop's court to the King's court for defect of justice, but none might appeal to the Papal court save with the King's consent. The privilege of sanctuary in churches or churchyards was repealed, so far as property and not persons was concerned.

After a passionate refusal the Primate at last gave his assent to the Constitutions; but this assent was soon retracted, and the King's savage resentment threw the moral advantage of the position into the Archbishop's hands. Vexatious charges were brought against him; in the Council of Northampton a few months later his life was said to be in danger, and all urged him to submit. But in the presence of danger the courage of the man rose to its full height. Grasping his archiepiscopal cross he entered the royal court, forbade the nobles to condemn him, and appealed to the Papal See. Shouts of " Traitor! traitor! " followed him as he retired. The Primate turned fiercely at the word: "Were I a knight," he retorted, "my sword should answer that foul taunt!" At nightfall he fled in disguise, and reached France through Flanders. For six years the contest raged bitterly; at Rome, at Paris, the agents of the two powers intrigued against each other. Henry stooped to acts of the meanest persecution in driving the Primate's kinsmen from England, and in threats to confiscate the lands of the Cistercians that he mightforce the monks of Pontigny to refuse Thomas a home; while Beket himself exhausted the patience of his friends by his violence and excommunications, as well as by the stubbornness with which he clung to the offensive clause "Saving the honour of my order," the addition of which would have practically neutralized the King's reforms.

The Pope counselled mildness, the French king for a time withdrew his support, his own clerks gave way at last. "Come up," said one of them bitterly when his horse stumbled on the road, "saving the honour of the Church and my order." But neither warning nor desertion moved the resolution of the Primate. Henry, in dread of papal excommunication, resolved at last on the coronation of his son, in defiance of the privileges of Canterbury, by the Archbishop of York. But the Pope's hands were now freed by his successes in Italy, and his threat of an interdict forced the king to a show of submission. The Archbishop was allowed to return after a reconciliation with Henry at Fréteval, and the Kentishmen flocked around him with uproarious welcome as he entered Canterbury. "This is England," said his clerks, as they saw the white headlands of the coast. "You will wish yourself elsewhere before fifty days are gone," said Thomas sadly, and his foreboding showed his appreciation of Henry's character. He was now in the royal power, and orders had already been issued in the younger Henry's name for his arrest, when four knights from the King's court, spurred to outrage by a passionate outburst of their master's wrath, crossed the sea and forced their way into the Archbishop's palace.