sought to wrest from William the western part of Normandy, the
older and the more thoroughly French part. No attack seems to have
been designed on the Bessin or the Cotentin. William was to be
allowed to keep those parts of his duchy, against which he had to
fight when the King was his ally at Val-es-dunes.

The two armies entered Normandy; that which was to act on the left
of the Seine was led by the King, the other by his brother Odo.
Against the King William made ready to act himself; eastern
Normandy was left to its own loyal nobles. But all Normandy was
now loyal; the men of the Saxon and Danish lands were as ready to
fight for their duke against the King as they had been to fight
against King and Duke together. But William avoided pitched
battles; indeed pitched battles are rare in the continental warfare
of the time. War consists largely in surprises, and still more in
the attack and defence of fortified places. The plan of William's
present campaign was wholly defensive; provisions and cattle were
to be carried out of the French line of march; the Duke on his
side, the other Norman leaders on the other side, were to watch the
enemy and attack them at any favourable moment. The commanders
east of the Seine, Count Robert of Eu, Hugh of Gournay, William
Crispin, and Walter Giffard, found their opportunity when the
French had entered the unfortified town of Mortemer and had given
themselves up to revelry. Fire and sword did the work. The whole
French army was slain, scattered, or taken prisoners. Ode escaped;
Guy of Ponthieu was taken. The Duke's success was still easier.
The tale runs that the news from Mortemer, suddenly announced to
the King's army in the dead of the night, struck them with panic,
and led to a hasty retreat out of the land.