At the beginning of their marriage, to be fair, Edward had relied on his French in-laws for help, such as Isabella's uncle Louis of Évreux (who advised trying for a peaceful solution to the split among his barons after Gaveston's death). Edward even trusted Isabella with the Great Seal on occasion, as you will see.
During earlier problems with the barons, Isabella had gone to France to assure that the French would come to Edward's aid if a civil conflict broke out. Later tensions in England however, both political and personal, motivated Isabella to act against her husband. The Despensers convinced Edward to take control of the lands in Isabella's possession in 1324, giving him the taxes and leaving Isabella dependent on the king for her finances. She was forced to trim her retinue of many retainers and friends. Worse for Isabella, one of her ladies-in-waiting was Eleanor de Clare. A niece of Edward, Eleanor had been married to Hugh the Younger, making her a spy in Isabella's household, reporting on her communications.
As of 1321 she had still supported her husband and his faction (including the Despensers) against the faction led by Thomas of Lancaster. On a pilgrimage to Canterbury, she stopped by Leeds Castle, held by the king's steward who had allied himself with Lancaster. The steward was away, and his wife refused entry to the queen, causing a fight to break out between the two groups and resulting in the death of some of Isabella's guards. Historians believe this was staged as a casus belli, giving Edward a reason to answer the royal insult by besieging Leeds and teaching a lesson to his steward—on this occasion he left Isabella with the Great Seal and in charge of Chancery—placing the steward's wife and children in the Tower and executing 13 of the Leeds garrison.
This was the start of the Despenser War, in which the king and his allies fought barons led by Humphrey de Bohun and Roger Mortimer. It was relatively brief; within a year, Lancaster was captured and executed and the rest forced to surrender. Mortimer was sentenced to death, but the sentence was commuted to life imprisonment in the Tower. In August 1323, however, he escaped and fled to France. Warrants were put out for his return, dead or alive.
Isabella's brother Charles was now King Charles IV of France, and he seized England's possessions in France. Isabella went to France with Edward's blessing, ostensibly to ask her brother for peace. At Charles' court, however, she found none other than Roger Mortimer, who had been not long before on the "other side" of her political position. From helping her husband start a war to deal with his enemies, to becoming his enemy because she saw Edward's rule becoming increasingly inappropriate, seemed to be an easy shift for her.
The alliance between Isabella and Mortimer led to deposing Edward and crowning her son; details tomorrow.