These things seem to have been done toward the end of August.  On the 27th and 28th of that month John was at Montfort, a castle some five-and-twenty miles from Rouen, held by one of his few faithful barons, Hugh of Gournay.  On the 30th, if not the 29th, he and all his available forces were back at Rouen, ready to attempt on that very night the relief of Les Andelys.  The King’s plan was a masterpiece of ingenuity; and the fact that the elaborate preparations needed for its execution were made so rapidly and so secretly as to escape detection by an enemy so close at hand goes far to show how mistaken are the charges of sloth and incapacity which, even in his own day, men brought against “John Softsword."[36]

He had arranged that a force of three hundred knights, three thousand mounted men-at-arms, and four thousand foot, under the command of William the Marshal, with a band of mercenaries under Lou Pescaire, should march by night from Rouen along the left bank of the Seine, and fall, under cover of darkness, upon the portion of the French army which still lay on that side of the river.  Meanwhile, seventy transport vessels, which had been built by Richard to serve either for sea or river traffic, and as many more boats as could be collected, were to be laden with provisions for the distressed garrison of the island fort, and convoyed up the stream by a flotilla of small warships, manned by “pirates” under a chief named Alan and carrying, besides their own daring and reckless crews, a force of three thousand Flemings.  Two hundred strokes of the oar, John reckoned, would bring these ships to the French pontoon; they must break it if they could; if not, they could at least cooeperate with the Marshal and Lou Pescaire in cutting off the northern division of the French host from its comrades and supplies on the left bank, and throw into the island fort provisions which would enable it to hold out till John himself should come to its rescue.

One error brought the scheme to ruin, an error neither of strategy nor of conduct, but of scientific knowledge.  John had miscalculated the time at which, on that night, the Seine would be navigable upstream, and his counsellors evidently shared his mistake till it was brought home to them by experience.  The land forces achieved their march without hinderance, and at the appointed hour, shortly before daybreak, fell upon the French camp with such a sudden and furious onslaught that the whole of its occupants fled across the pontoon, which broke under their weight.  But the fleet, which had been intended to arrive at the same time, was unable to make way against the tide, and before it could reach its destination the French had rallied on the northern bank, repaired the pontoon, recrossed it in full force, and routed John’s troops.  The ships, when they at last came up, thus found themselves unsupported in their turn, and though they made a gallant fight they were beaten back with heavy loss.  In the flush of victory one young Frenchman contrived to set fire to the island fort; it surrendered, and the whole population of the New Andely fled in a panic to Chateau Gaillard, leaving their town to be occupied by Philip.