To this day, 950 years after the Norman Conquest, many of us are still fascinated by the causes of this pivotal event—and I am one of them. If Harold Godwineson hadn't been 260 miles away fighting his brother when Duke William landed at Pevensey, things might have gone differently. So where does Tostig come into this? From outlawry to Stamford Bridge, Tostig was on the wrong side of the law. In that critical battle, he seems to have been second in command after Harald Hardrada and has been branded as a traitor ever since.
It was thought by many that Tostig himself persuaded Hardrada to invade, thus forcing King Harold to rush north and defend his kingdom against the Vikings. However, this conclusion is by no means certain; nobody was tracing Tostig's movements in the early part of 1066. It's entirely possible that the Norwegian King planned the invasion on his own, and Tostig merely fell in with his army when the time came. There is no doubt that Hardrada was the leader of the Viking invasion. What exactly Tostig thought to accomplish is uncertain. Perhaps he only wanted his old earldom back. Or, he might have bargained to rule his old earldom as sub-king to Harald. Maybe he hoped Hardrada would get killed and he could rule in his stead, unlikely though that sounds.