British troops invaded Waikato by crossing the Mangatāwhiri Stream, which the Kīngitanga (Māori King movement) had declared an aukati (a line not to be crossed).
The Kīngitanga had been formally established in 1858. The government saw its refusal to sell land as an impediment to European settlement. Kīngitanga warriors fought in Taranaki in 1860–61, fuelling fears that the movement posed a challenge to British sovereignty. In January 1863, Governor George Grey announced his intention to dig around the Kīngitanga until it fell.
Amid rumours of an imminent Maori attack on Auckland from Waikato, settlers and missionaries fled north. Grey exploited the situation to persuade the British authorities to send him thousands more soldiers.
When fighting resumed in Taranaki in 1863, the alleged involvement of Kīngitanga forces gave Grey the excuse he needed. In July, he gave Māori living between Auckland and the Waikato River an ultimatum: swear allegiance to the Queen or be deemed rebels.
Within two days, Lieutenant-General Duncan Cameron had crossed the Mangatāwhiri Stream with the declared intention of establishing military posts on the Waikato River. Five days later, the first battle of the Waikato War was fought at Koheroa, near Mercer.