Complex sentence: According to Fernand Braudel, the most important event of Walloon historyand that of Belgian historyis the Barbarian invasions, which he says is an interesting example of the "longue durée" event.
Split into simpler sentences: According to Fernand Braudel himself, the most important event of Walloon history (and of Belgian history) is the so-called Barbarian invasions. For the great French historian, this is one interesting example of the "longue durée" event.