Alarmed for the safety of his adherents, Robert, without hesitation, came over to England, accompanied only by a small escort, and placed himself unreservedly in his brother's power for the purpose of pleading the cause of the proscribed nobles.
At this time Robert resigned his pension of two thousand pounds. According to some historians, he was detained by Henry as a prisoner, and the pension was the price paid by the duke for his liberty; while another account states that the sum was given as a present to the Queen Matilda. It is, however, certain that Robert soon returned to Normandy without having succeeded in the object of his visit.
The Duke of Normandy was ill-fitted to restrain the excesses of his turbulent barons, or to hold with a firm hand the reins of government. Many disorders sprang up in his duchy, and were left unnoticed or unpunished by the sovereign. The fair Sibylla died in 1102, and since that time the duke had resumed his irregular way of life, and had shown more completely than ever his utter incapacity for the management of public affairs.
King Henry took advantage of this state of things to interfere in the disputes of the Norman barons; and, after appearing for a time in the character of a mediator, he at length threw off the mask, and declared himself the protector of the duchy against the maladministration of his brother. He summoned Robert to give up possession of the duchy in return for an annual payment of money. The duke indignantly refused to comply with the demand, and Henry prepared to dispossess his brother by force.
In the year 1105 the king entered Normandy with an army, and captured several castles and fortified places. Robert, however, was not without means of defence; some few nobles of power and influence still remained attached to his cause, and Henry returned to England, having added Caen and Bayeux to his possessions.
A second campaign was opened in the following year, and Henry crossed the Channel with a more formidable armament than before. He appeared before Tenchebrai, an important stronghold, situated at a few leagues' distance from Mortain. Having in vain attempted to corrupt the garrison with gold, the king laid siege to the castle with his whole army. Messengers came to Robert with the news that his troops were hard pressed by the enemy, and the duke promised that, in defiance of every obstacle, he would come on a certain day to their assistance. The promise was redeemed; and, at the time appointed, the duke, with a small but gallant band of troops, attacked the army of his brother. Placing himself at the head of his knights he dashed in upon the English infantry, which gave way before him in disorder. So impetuous was the charge, that the fortune of the day seemed likely to be in favour of Robert, when the cowardice or treachery of the Earl of Shrewsbury turned the tide of affairs. De Belesme, whose troops formed an important division of the army of the duke, suddenly fled from the field. A panic ensued among the Normans, and the brilliant deeds of valour performed by their leader failed to restore their courage or to secure the victory. After a desperate resistance, Robert was taken prisoner, with many of the chief nobles who had fought under his banner.