In the late 1850s, Tamihana was largely responsible for the establishment of the Māori King Movement, which aimed to unify rebel Māori by setting up a kingship in opposition to the British government. He was able to persuade several iwi to join the movement, and Potatau Te Wherowhero of Ngāti Mahuta to take on the role of first King. Tamihana provided a statement of laws, based on the Bible. Although the movement was seen by many Pākehā and the government as rebellion, Tamihana intended that the Māori King would be in alliance with Queen Victoria. Tamihana became a diplomat and publicist for the movement, founding a Māori language newspaper for it. In 1861 Governor Thomas Gore Browne issued a declaration demanding Māori submission to the British Crown. Tamihana wrote to him explaining that the King Movement was not in conflict with the Queen but refused to swear the oath of allegiance. He expressed concern that the Governor seemed intent on war but failed to see the implications of rebellion. Later, he wrote a series of 14 threatening letters to Grey who realised that Tamihana was backed by the fierce Rewi Maniapoto. When war did break out, after the killing of 7 British soldiers in a time of peace in Taranaki and the attempted murder of Gorst, a government agent at Te Awamutu, Tamihana remained in favour of negotiation, but others within the King Movement, such as Rewi Maniapoto preferred to fight. Throughout the Invasion of the Waikato Tamihana attempted to negotiate with government forces, to little effect. After the war he campaigned against the resultant confiscation of land.