In the winter of 1464, the Swedish bishop Kettil Karlsson (Vasa) instigated an uprising against King Christian I of Oldenburg who at the time ruled all of Scandinavia. In order to crush the uprising the king quickly responded with a war campaign against the insurgents. In April his army was ambushed in Hälla forest in Västmanland by forces commanded by the bishop. In the battle that followed the king was defeated and fled back to Stockholm. The study of medieval warfare in Sweden has hitherto yielded little interest among scholars. The aim of this article is thus to add to this subject by investigating these events from a military point of view. The article address the question of the actual geographical location of the battle, what elements the two clash-ing armies composed of, further, what really happened during the course of the battle and finally how the battle ended and what the military consequences were.