1325-6: THE WAR OF SAINT-SARDOS
"For France, the year began with a wedding. Charles d'Evereux, eldest son of Louis, Count of Evereux and heir to the lordship of Etampes, wed Marie of Luxembourg, sister to King John of Bohemia, in a lavish ceremony in Paris[1] meant to symbolize the power and wealth of the French throne. If this showed any thoughts about the upcoming conflict with England, it was contempt--a later (somewhat doubtful) story had the Regent stating that he considered the English little more than "an infestation of vermin". If this was his opinion then Edward gave him little reason to change it--the King's initial response to the sack of Saint-Sardos was to insist that he had had nothing to do with it. Ralph Bennett, the offending Seneschal of Gascony that the French believed to have met with prominent Gascon noble Raymond-Bernard of Monpezat, ringleader and architect of the sack shortly before it occurred, was recalled, and an embassy sent, lead by the King's youngest brother, Edmund of Woodstock, Earl of Kent[2].
"By all accounts, the embassy went poorly--England's claims and excuses were treated with skepticism, and the Regent's outstanding order to muster at the border of Aquitaine in June remained active. Edward seems to have hoped that his new father-in-law might spring to his aide, but Louis was preoccupied by the matter of his own excommunication, and settling affairs in Germany, and planning his own invasion of Italy as a response. As it became rapidly became clearer that the Emperor was not coming to England's aid, Kent appears to have handled matters by giving into the Regent's demands for the Lord of Monpezat. However, upon reaching Gascony, the young Earl seems to have changed his mind, and sent the French officials back empty-handed. It would be war.
"In August, the French army invaded the duchy, the Count of Valois at its head. They found a land guarded by skeleton garrisons and underfunded troops. Within six weeks, France had almost completely occupied Gascony. The Earl of Kent had managed a few weeks of spirited resistance at La Reole, an underwhelming high point for English arms during the entire squalid mess[3]. The English, in between a rounds of apportioning blame amongst their commanders, considered various unlikely schemes of relief before woefully heading to the table. In France, celebrations of the rapid victory would be cut short, when late in December, Charles of Valois fell victim to a stroke. He would expire early in January[4], leaving the peace negotiations--and the Regency--to his eldest son, Philip..."
--John I of France, Vol. 1; A King in His Cradle, Antony Oates (1978)
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[1] IOTL, Marie would wed Charles IV of France. While this might be considered a step down, it should be remembered that Charles is a biggish wheel in France, and Marie's brother isn't even Emperor.
[2] Edmund of Kent handled things IOTL as well. This was not the start of a glorious career.
[3] This how the war ran IOTL. The Hundred Years War, it wasn't.
[4] IOTL, Charles died in December of 1325.