778 is a year of legend.
The bottom left corner of the Frankish Empire was connected to the top right corner of the Iberian Peninsula, most of which was Moslem. But the top area itself was an independent Catholic kingdom. We're not sure exactly what happened, but it was something like this:
Despite having much of his army tied up fighting the Saxons, Charlemagne had invaded Moslem Iberia (modern Spain). Surprisingly, he had Moslem allies who were promised governing positions if the Franks won. Advancing rapidly through thick forest and capturing large areas with little loss, the Franks got bogged down at the city of Zaragoza, and after a year-long siege, Charlemagne ordered his army to retreat. As they made their way through the thick forestland, the Franks were ambushed by Catholic Basques, an ethnic group from northern Iberia, who apparently wanted to plunder the gold in the Franks' baggage train.
The Basques massacred most of the rear guard, plundered the baggage train, and fled into the forest. Unable to pursue them, Charlemagne and his devastated army returned to their own kingdom. But two centuries later, "The Song of Roland" became the first great piece of French literature. It tells an incredible story of how Roland, commander of the rear guard, led a fantastic defense against a gigantic Moslem army before being killed. The epic poem became a classic story of the battle between Catholicism and Islam, and it is indeed an inspiring tale of heroism, magic, and religion. It isn't true, but it is the foundation of the modern French language. I had to study it in grade schol, and it really is a great tale.
And speaking of legends, Widukind returns to Saxony from Denmark and begins establishing a rebellion against the Franks.
On the other side of Europe, the Byzantine Empire wins a series of victories over the Moslems and retakes some of its lost territory.