See plans of the battle which appear on pp. xiv-xv of this book.
By 23 July 1214, Otto and his allies were gathering their army at Valenciennes. Philip marched north into Flanders with fire and sword, reaching Tournai on 26 July with an army of about 1,400 knights and 5,000-6,000 foot. On the very same day his enemies encamped at Mortagne, some 12km to the south. They had about the same number of knights and perhaps as many as 7,500 foot. When the French and allied armies became aware of how close they were to one another, the leaders were forced to make decisions. 1
We do not know their precise intentions at this stage. However, the coalition needed to break Philip's power and prestige quickly, and so were probably seeking battle. On the night of 26 July, when Philip learned of the presence of the allies at Mortagne, he called a council of war. He considered an attack, but was advised that the ground near Mortagne was not suitable, and in any case he was reluctant to fight on the next day, which was a Sunday. It was decided to withdraw to Lille, 30km away, and so find another route by which to invade Hainaut. This account by Guillaume le Breton, the story of a precipitate retreat in the Anonymous of Béthune, and the subsequent behaviour of the king, all suggest that he really wanted to checkmate the allies rather than challenge them in the open field, in the hope that their army would fall apart. There was every prospect of this: Henry of Brabant had married his daughter to Otto in May 1214, but he had recently been in the pay of Philip, whose daughter Marie he had married in April 1213. His sons were held hostage by Ferrand of Flanders. On the other hand, Philip was suspicious of the Count of St Pol, who quipped bitterly as he led his forces into battle that he would “be a good traitor today”. 2 The whole coalition was cemented by King John's largesse, which in the end had limits, and this may well have been what Philip was counting on.