The reign of William Longsword is also rather obscure, at least until the end, when he suddenly came to play a major role in the conflicts among King Louis IV, Hugh the Great, Herbert II of Vermandois, and King Otto of the East Franks. We know nothing about his birth and upbringing, although it may be significant that unlike his father, who was always known by the Northmannic name Rollo even after his "baptism" under the name Robert, William is known to us only by a Frankish name. He was clearly born long before the agreement with Charles Simplex, but apparently Rollo had been Frankified enough already to give his son a Frankish name.
Dudo recounts one important event at the beginning of William's reign; no other source mentions it, which means Dudo's story should be taken with considerable scepticism, but for reasons I will discuss later, this account probably can be trusted, at least in its broad outline. In this tale, a Northman leader named Riulf rebels against William, claiming that William is too Frankish for Northmannic tastes, and that he intends to enrich his own relatives and his Frankish friends at the expense of the true Northmen. Riulf sends an envoy to William demanding "all the land up to the Risle." William replies that he cannot give Riulf the land (he doesn't say why), but offers to make Riulf in effect co-ruler, subordinate in name only. Riulf responds by moving an army towards Rouen, successfully crossing the Seine and camping outside the city. William then offers Riulf the land "not only up to the Risle, but all the way to the Seine." Riulf, smelling weakness, refuses the offer; William then contemplates flight to Francia, but is shamed by his men into combat. In what seems to have been a sneak attack, William falls upon Riulf's camp, scattering the rebel troops, killing many of them, and driving the rest (including Riulf himself) into flight.
Since Dudo is very much a partisan of the Rollonids, it seems somewhat odd that he would tell a story that reflects so badly on one of his heros. William here is rather cowardly, trying to cave in to Riulf at every opportunity and only being forced to put up a fight by the scorn of his men, and achieving victory through what appears not to be the manliest of strategies. It would seem that this account was known widely enough in Dudo's Rouen that he couldn't simply ignore it, which makes sense; Rouen prospered greatly through the Rollonid accommodation of both native Franks and immigrant Northmen, and if the city had been taken by hardcore, anti-Frankish Northmen, that prosperity would have been threatened. So Dudo simply had to put as good a face on it as possible, using it as the event that showed William what he was really made of. It shows, however, that even twenty years after the official establishment of the Rollonid principality, Rollonid rule even over the capital city was shaky, and other Northmannic leaders of equal power were nearby, waiting for the chance to overthrow the Franco-Northmannic regime of the Rollonids.
The only other mention of William's reign before his participation in the Frankish civil wars near the end of his life is Flodoard's entry on his meeting with King Ralph in 933, when William committed himself to Ralph and in return received "the land of the Bretons on the sea-shore." This has plausibly been identified as the Contentin and Avranchin, which had earlier been conceded to the Bretons, and means that by 933 the Rollonids had been given royal approval to do what they could in all of what would become Normandy. It does not, however, mean that they controlled all this territory, and in fact it would only be in the eleventh century that all Normandy fell under the power of the Rollonids (for example, it was not until 1030 that the bishops of Avranches felt safe returning to their see from Rouen, where they had lived in exile since the Northmannic incursions).
In 936 King Ralph died. His brother Hugh the Black succeeded him in Burgundy, but made no attempt to gain the throne for himself; there would be no Burgundian dynasty of West Frankish kings. Once again Hugh the Great was in position to put himself forward, but once again he did not do so. Instead, he recalled Charles Simplex's son Louis from his exile in England and arranged for his coronation; presumably, he believed that the fifteen-year-old Carolingian would be grateful, and subject to his control. As one of their first acts, Hugh and Louis went to Burgundy and secured the recognition of Louis by Ralph's brother (as well as territorial concessions to Hugh). But all did not go well for Hugh; in 937, Louis left Hugh's guardianship and set himself up at Laon. Over the next couple of years, Louis, Hugh the Great, Herbert II of Vermandois, Arnulf of Flanders, and other Frankish magnates began to jockey for position, as ephemeral alliances formed and dissolved, and minor military actions erupted.
Meanwhile, the detente that seems to have existed between the Rollonid Principality and Flanders since the end of Rollo's life suddenly shattered. As before, the flashpoint was the county of Ponthieu, located directly between the two rival states, and its capital of Montreuil. Arnulf had, through the 930s, expanded his power in the area, disinheriting his nephews and seizing the counties of Boulogne and the Ternois for himself. Arnulf then made an alliance with Herbert of Vermandois, marrying Herbert's sister; he now was secure on his southern border, which freed him to turn his attention to the Rollonid Principality.
But it was William Longsword who seems to have opened hostilities; after a career spent largely in obscurity, beginning in 939 William's forces raided Arnulf's territories. This resulted in William's excommunication. Arnulf responded to William's attack by invading Ponthieu and capturing Montreuil from Herluin, the count of Ponthieu. Herluin responded by going to Northmanland and raising a Northmannic army, with whose help he then attacked and recaptured Montreuil, slaughtering most of Arnulf's garrison. Arnulf, in turn, sent more of his men into Ponthieu to ravage Herluin's lands.
Meanwhile, a group of Lotharingian nobles revolted against King Otto; King Louis took advantage of this by traveling to Lotharingia and receiving the homage of the rebels. Otto responded by launching large-scale raids into West Francia; Louis did not have the resources to effectively oppose him. A remarkable coalition including Hugh the Great, Herbert of Vermandois, Arnulf of Flanders, and William Longsword met with Otto, agreeing to support him against Louis. After the death of Louis' leading ally in Lotharingia, he bit the bullet and ended his direct conflict with Otto by abandoning Lotharingia. This did not end the conspiracy against him, however; Hugh and Herbert went to the east to meet with Otto. This time, however, William Longsword refrained; instead, he traveled to Amiens for a meeting with King Louis. He "committed himself to the king," who then conceded to him the lands that King Charles had given to him. This would presumably be the "first grant," of Rouen and its environs; the legal status of the later grants by Charles' and Louis' rivals, the Robertines, is not at all clear. But although William did not take direct action against Louis, when Hugh and Herbert besieged Reims, intending to depose the archbishop who was Louis' chancellor and staunchest supporter, William joined them. The town was captured and the archbishop expelled; Herbert's son, who had earlier been archbishop and subsequently deposed, was now restored. But when Herbert and Hugh went on to besiege the royal stronghold of Laon, William did not join them.
Louis succeeded in forcing his foes to abandon their attempt on Laon. He and Otto then traded expeditions into each other's lands; Louis' vigorous challenge to Otto seems to have intimidated Hugh and Herbert, who stayed out of the fray. But Louis did not succeed at pressing his advantage, and in 941 Hugh and Herbert attacked Laon again, for the first time taking arms directly against the king. This time, their siege was successful, and Louis barely managed to escape. Shortly after, Hugh, Herbert, William Longsword, and Arnulf met; we do not know what they discussed, but if Hugh and Herbert were trying to get William and Arnulf to take a more active stance against the king, they failed. William and Arnulf continued to stay out of the fray. Louis seemed to have taken heart at William's restraint, and in 942 he sent Roger, count of Laon, to Rouen as a royal envoy. Roger died at Rouen, but not before negotiating a new peace between William and the king. Louis then traveled to Rouen to seal the alliance personally, where he was "received in royal fashion." Louis and William then negotiated peace with King Otto, depriving Hugh and Herbert of their chief foreign supporter.
It was at this moment that Arnulf sent messengers to William Longsword, saying that he wanted to settle their conflict over Montreuil. William went to the meeting, where he was murdered by Arnulf's men.
William's career began in danger, as the principality his father had built was almost destroyed by a Northmannic revolt against a son seen as tied too strongly to Frankish interests. Barely surviving this revolt, William kept a low profile during most of his reign, emerging briefly from obscurity only once, to receive a grant of the Cotentin and the Avranchin (in reality, probably only permission to do what he could there) from representatives of King Ralph. At the end of his life, his conflicts with Arnulf of Flanders over Montreuil quickly drew him into the conflict between King Louis and his greatest nobles, Hugh and Herbert. William was playing at the highest level of the Frankish political world, receiving the friendship of the king. But perhaps this relationship made him overconfident; when he was invited to a meeting with his oldest enemy, he was too trusting, and this trust led directly to his death. And the murder of William Longsword revealed just how fragile the Rollonid Principality remained.