Denmark had been coming apart since Eric III abdicated in 1146. Sweyn, son of Eric Emune, was proclaimed king in Zealand and Scania; Canute, son of King Magnus, was proclaimed king in Jutland. Canute tried twice to take Zealand, in 1147 and again in 1150, was beaten off both times, and went to Germany to raise troops. He lost again at Gedebaek near Viborg in 1152 and appealed over Sweyn's head to the German king Frederick I Barbarossa, who summoned both men to Merseburg and confirmed Sweyn in his kingship - at the price of Sweyn swearing fealty to him. That price was the miscalculation. Danish nobles who could tolerate a civil war could not tolerate a German overlord. Valdemar, who had thrown in with Sweyn and been made duke of Schleswig for it, changed sides and was betrothed to Canute's half-sister Sophie. By 1154 both Canute and Sweyn were being hailed as king at the assembly in Viborg.

The settlement, when it finally came, was a partition: Valdemar would hold Jutland, Canute the islands of Zealand and Funen, Sweyn Scania. To seal it the three met at Roskilde on 9 August 1157. What happened there is known chiefly through Saxo Grammaticus, writing decades afterwards for Valdemar's dynasty, which is worth remembering - but no version of the night ends without Canute dead on the floor of the hall. He was a young man who had spent his entire adult life trying to hold a claim he had inherited, and he was murdered under a truce he had agreed to in good faith. Valdemar took a wound and ran.

Treachery is expensive. When word of the feast spread, men came to Valdemar in numbers, and he raised a substantial army in Jutland while Sweyn crossed to meet him. Sweyn put ashore at Grena, at the mouth of the Djursa, and lost his fleet there to a combination of force and trickery. He pushed inland towards Randers. Valdemar pulled back over the Gudena and tore down the bridge behind him, buying weeks. Not until the end of September did he judge his host strong enough to stand, and the two armies did not come together until 23 October, on Grathe Heath, in the flat open country of central Jutland.

The battle did not last long. Sweyn could not find Valdemar's main body, and when it hit him it hit hard enough that he broke away from his own army and rode. He went the wrong way, into the marshy ground at one end of Hauge Lake, where he shed his weapons and armour to get clear of the mire. He did not get clear. He was taken alive, and tradition holds that he was killed with an axe by peasants - men from the district whose fields and harvests eleven years of dynastic war had been fought across. Whatever one thinks of Sweyn, and he had murdered a guest at his own table, it was a squalid end for a man who had been crowned. He is remembered by the name of the ground: Svend Grathe.

Valdemar had outlived every rival and was now sole king of a wrecked country. He purged Sweyn's supporters, then spent the rest of his reign rebuilding, and history gave him the name Valdemar the Great. The heath itself is mostly farmland and plantation now, unremarkable to look at, and easy to drive past. In 1892 the Danish writer Thor Lange put up a stone cross on the site - a small, deliberate marker for a field where a civil war stopped.