The causes and the stages of the development of the Mercian power, and even the origin of the Mercian state, are alike hidden from us. All that can be said is that in the early part of the seventh century we find the Mercians, an Anglian tribe, manifesting themselves in force in Staffordshire and Shropshire along the Welsh _March_ from which they perhaps derived their name. As the century proceeds, they conquer or ally with themselves the Middle Anglians, who seem to have inhabited Leicestershire and some of the country adjacent thereto; as well as the South Angles in Buckinghamshire, Bedfordshire, and Hertfordshire, who sooner or later became incorporated in the new state. The agent in these great changes was probably Penda himself, the strong-willed pagan who, in 626, at the age of fifty, ascended the Mercian throne, which he occupied for nearly thirty years. Of his alliance with Cadwallon of Wales, and his successful wars with the Northumbrian kings, Edwin and Oswald, enough has already been said in previous chapters; but his dealings with Wessex and East Anglia require some further notice. In the year 628, as we learn from the Chronicle, Cynegils and Cwichelm fought with Penda at Cirencester and made a treaty there. These are the two Kings of Wessex, apparently reigning together as father and son, who sent the assassin to deal that murderous blow at the life of Edwin which was foiled only by the self-devotion of the loyal thegn, Lilla. That event and the retaliatory campaign of Edwin against Wessex no doubt preceded by some years this war of 628 between Wessex and Mercia. Of the details of the treaty by which the war was ended we know nothing, but it has been conjectured with some probability[67] that it included a cession of the north-western conquests of Ceawlin to Mercia, and the acceptance by Wessex of the line of the Thames as her northern boundary.[68]