In 764 King Eanmund of Kent died and was replaced by two joint kings: Heabert and Egbert II.
Offa himself travelled down to Canterbury in 764 to attend the coronations. The method by which the people of Kent chose a new king is not entirely clear, but it seems to have been the nobles who had the right to appoint a new ruler, though they had to choose an adult man from among the ranks of the ruling dynasty, the Eskings. Kent had seen joint kings before, there being a long standing division between east Kent and west Kent, with the River Medway marking the boundary. Usually one of the joint kings was the acknowledged senior with the junior ruler concentrating on his own lands and steering clear of international or church matters.
A coin of Offa showing him in armour, but bareheaded. The inscription describes him as "Rex Mercior" or King of Mercia.
There is some evidence that it was Offa who had chosen the new joint kings. It may be that he came to Kent not to attend the celebrations but to ensure that his placemen got chosen.
While he was there, Offa chose to demonstrate his overlordship of Kent in a very solid fashion. King Heabert granted some land to the Bishop of Rochester. It was usual when this happened for the title deeds, or charter, to be signed by the ruling King and witnessed by whichever nobles and clerics were around at the time and had learned to read and write well enough to sign their names and, hopefully, have read the charter. This particular charter, however, was signed not by Heabert, but by Offa. Heabert signed as a mere witness among others though he did describe himself as King of Kent when he did so. One name noticeably absent was that of the new King Egbert II.
When Egbert did choose to act he did so on his own account. Some months after Offa had gone back to Mercia, Egbert chose to have a charter confirmed by King Heabert, implying that it was Heabert who was the senior ruler. Offa's name is nowhere to be seen. One name that does appear as a witness is that of Jaenbert, Abbot of St Augustine's Abbey in Canterbury. The following year Bregowine, Archbishop of Canterbury, died and Jaenbert was elected to succeed him. A few months later Heabert died and Egbert II became sole King of Kent.
All appeared calm, with Offa clearly having the upper hand over Kent. But ten years later he overstepped the mark. In 774 Offa called a meeting of Mercian churchmen and Archbishop Jaenbert went along to officiate as was his right. Travelling with Jaenbert was a nobleman named Baban, who may have been acting as observer for Egbert II or who may have been there on his own business.
At the meeting Offa decided to give some land he owned to the Archbishopric of Canterbury. Jaenbert was, naturally, pleased but Baban was not. Although the land belonged to Offa the taxes on it were owed to Egbert II and so Offa should have asked Egbert for permission to transfer the land. He did not, he simply signed the charter and when Baban objected told him that Egbert would have to accept the situation.
When the news reached Egbert he was predictably furious. Offa had acted as if he, not Egbert, were King of Kent. Having an overlord living miles away in Mercia was one thing. Having an overlord who issued orders to you, expected to have them obeyed and then insulted your dignity was something quite different.
It was not only Egbert who was enraged, the Kentish nobles were incensed as well. Egbert was from their own royal family, was related to many of them and had been chosen by them. Offa was a foreigner with no links to Kent at all. While they could expect to have some influence over a local king, they could have none over Offa. Jaenbert was also unimpressed. It suited the Archbishops of Canterbury to be ruled by a local king who could be influenced by them rather than to have a powerful but remote monarch.
In 775 Egbert threw off Offa's overlordship. He began ruling in his own right, making no reference to Offa and not seeking to have new laws or taxes approved by his supposed overlord. In his turn Offa could not afford to let this rebellion, as he saw it, go unpunished. He gathered an army and in 776 sent it south to invade Kent.