The Viking king attacked first. Harold rushed up to fight him at York. He destroyed the Viking army.
At the same time, William of Normandy was sailing to the south coast, so Harold had to rush back to fight him.
Harold's men were now exhausted, and although they fought bravely, they were eventually led into making mistakes. This let William's army win. It was the end of Anglo-Saxon times and the start of the age of the Normans…