Treaty of Athis-sur-Orge The Franco-Flemish War (French: Guerre de Flandre; Dutch: Vlaamse opstand) was a conflict between the Kingdom of France and the County of Flanders between 1297 and 1305.

Meanwhile, despite having been forced to accept the loss of Normandy, Maine, Anjou and Poitou in the 1259 Treaty of Paris after a century of conflict, the Kingdom of England continued to hold the Channel Islands and the massive Duchy of Aquitaine in nominal vassalage to the French king.

Philip then ordered the arrest and imprisonment of Guy and two of his sons; the marriage was called off and Philippa was brought to Paris, where she remained until her death in 1306.

Philip responded by declaring Guy's titles forfeit, annexing Flanders to the royal domain, and sending a French army under Robert II of Artois to conquer it.

The count's eldest son Robert of Bethune speedily occupied Mortagne, at the confluence of the Scheldt and the Scarpe, and the castle of Helkijn.

During the siege, French raiding parties marched through the Flemish countryside, burning or conquering the towns of Komen, Waasten, and Kortrijk, which surrendered to Valois.

Five days later Lille surrendered to King Philip and the 3,000-man strong Flemish Army, led by Robert of Bethune, was allowed to march out to Roeselare.

Although facing problems at home, at the end of August 1297, King Edward eventually moved an army of 895 knights and 7,560 infantry and bowmen to Flanders.

During this period, negotiations between the French and English kings and the other warring parties—including Count Guy—took place before papal envoys, while the combatants simultaneously strengthened the defences of the Flemish towns in their hands.

From March 1300 the French besieged Damme and Ypres, where the defences respectively was led by the Count's sons William of Dendermonde and Guy of Namur.

By mid-May the old count, his sons Robert and William and several Flemish nobles were led into captivity in France, and the whole of Flanders was under French control.

The urban proletariat hoped for more justice and a better distribution of wealth under the new ruler, but Philip IV appointed Jacques de Châtillon as governor of the county, a very bad choice.

When the Flemish besieged Kortrijk on 9 and 10 July, a powerful French army led by Count Robert II of Artois arrived to crush the rebellion.

Guy of Namur, son of the Count of Flanders, formed a fleet at Sluis and sailed on 23 April 1303 to claim Zeeland for the Flemish.

After some initial successes, Guy was defeated on 10 and 11 August 1304 in the Battle of Zierikzee by a combined Franco-Hollandic fleet under Rainier Grimaldi, who had been sent by Philip IV of France to aid the Count of Holland.

After further minor battles, the Treaty of Athis-sur-Orge was eventually signed on 23 June 1305, which recognised Flemish independence as a fief but at the cost of the cities of Lille, Douai and Bethune, which were transferred to the French crown-lands, and the paying of exorbitant fines to Philip IV.