The tide of war flowed to the southern march of Wales.  Llewelyn and Richard Marshal devastated Glamorgan, which, as a part of the Gloucester inheritance, was under the custody of the Bishop of Winchester.  They took nearly all its castles, including that of Cardiff.  Thence they subdued Usk, Abergavenny, and other neighbouring strongholds, while an independent army, including the marshal’s Pembrokeshire vassals and the men of the princes of South Wales, wasted months in a vain attack on Carmarthen.  The king’s vassals were again summoned to Gloucester, whence Henry led them early in November towards Chepstow, the centre of the marshal’s estates in Gwent.  Earl Richard devastated his lands so effectively that the king could not support his army on them, and was compelled to move up the Wye valley towards the castles of Monmouth, Skenfrith, Whitecastle, and Grosmont, the strong quadrilateral of Upper Gwent which still remained in the hands of the king’s friends.  Marching to the most remote of these, Grosmont, on the upper Monnow, Henry spent several days in the castle, while his army lay around under canvas.  On the night of November 11, the sleeping soldiers were suddenly set upon by the barons and their Welsh allies; they fled unarmed to the castle, or scattered in confusion.  The assailants seized their horses, harness, arms and provisions, but refrained from slaying or capturing them.  The royal forces never rallied.  Many gladly went home, giving as their excuse that they were unable to fight since they had lost their equipment.  Henry and his ministers withdrew to Gloucester.  More convinced than ever of the treachery of Englishmen, the king entrusted the defence of the border castles to mercenaries from Poitou.

The fighting centred round Monmouth, which Richard approached on the 25th with a small company.  A sudden sortie almost overwhelmed the little band.  The marshal held his own heroically against twelve, until at last Baldwin of Guines, the warden of the castle, took him prisoner.  Thereupon Baldwin fell to the ground, his armour pierced by a lucky bolt from a crossbow.  His followers, smitten with panic, abandoned the marshal, and bore their leader home.  By that time, however, the bulk of the marshal’s forces had come upon the scene.  A general engagement followed, in which the Anglo-Welsh army drove the enemy back into Monmouth and took possession of the castle.  This set the marshal free to march northwards and join Llewelyn in a vigorous attack upon Shrewsbury.  In January, 1234, they burnt that town and retired to their own lands loaded with booty.  Meanwhile Siward devastated the estates of the Poitevins and of Richard of Cornwall.  Afraid to be cut off from his retreat to England the king abandoned Gloucester, where he had kept his melancholy Christmas court, and found a surer refuge in Bishop Peter’s cathedral city.  Thereupon Gloucestershire suffered the fate of Shropshire.  “It was a wretched sight for travellers in that region to see on the highways innumerable dead bodies lying naked and unburied, to be devoured by birds of prey, and so polluting the air that they infected healthy men with mortal sickness."[1]