A quick look, today, at the role Arnulf played in the English Civil War. From the beginning, Arnulf followed the example of the bishops of Normandy in supporting King Stephen against the claims of Matilda. Stephen was, of course, a Frenchman, but then too, so was Matilda's husband Geoffrey of Anjou. Arnulf, hoping to become a bishop himself, knew he needed the support of the ruler of Normandy, and he followed the guidance of his uncle, John of Lisieux.
In 1139, the two claimants to the English throne brought their case before the pope at the Second Lateran Council. Because of his growing reputation as an attack dog, Arnulf was asked to argue the case against Matilda. There is no record of what he said, but letters from those who were there make it pretty clear.
Arnulf called Matilda a royal bastard, just like her 22 other illegitimate siblings. Why? Well, he said, Henry I found his wife, Maud, in a nunnery . But since he needed a wife with Anglo-Saxon blood, he forced her to leave and marry him. She had taken final vows as a nun, Arnulf assured his listeners. Therefore, the marriage was illegal and Matilda, the only child of that union, was not Henry's legal heir. Was it true? Historians still argue the case. For Arnulf, it didn't matter. The charge was simply a rhetorical trick that allowed him to use church teachings against a political foe. (Does any of that sound familiar today?)
Pope Innocent did not rule in favor of one or the other side but simply dismissed them, but he later gave tacit approval to Stephen by recognizing his occupation of England. Did that mean Arnulf had chosen the right side? Not so fast! In Normandy, Geoffrey of Anjou laid liege to the city of Lisieux. Bishop John set fire to the city rather than surrender to Geoffrey, but his defiance did little good. When John died in 1141, Arnulf was in line for the bishopric, but Geoffrey prevented him from taking his seat until he paid a huge fine.
And then, of course, the two sides found a way to settle their own dispute, in a way that left Matilda's son Henry as the lawful king of England and ruler of Normandy. You might guess that the young Henry II was not amused by the bishop who had publicly called his mother a bastard.