Thomas Becket was born around 1120 in London to a family of Norman merchant stock, rose through clerical administration into royal service, and became Lord Chancellor of England in 1155 under King Henry II, whose close friend and most effective instrument of government he was for the next seven years. Their partnership was close enough that when Henry engineered Becket's appointment as Archbishop of Canterbury in 1162, he expected to gain in the church's highest English office a loyal ally who would help him bring ecclesiastical courts and privileges more firmly under royal control.
Becket instead resigned the chancellorship and adopted the role of archbishop with what contemporaries described as unexpected and total seriousness, defending the church's independent jurisdiction, particularly its right to try clergy in its own courts rather than the king's, with the same single-minded intensity he had once brought to royal service. The resulting conflict over the Constitutions of Clarendon, Henry's 1164 attempt to formalize royal authority over the church, drove Becket into six years of exile in France, from which he returned to Canterbury in late 1170 still refusing to yield the church's claims.