In September 1325, Edward II sent his twelve-year-old son Edward of Windsor, earl of Chester and now duke of Aquitaine and count of Ponthieu, to France to perform homage to Charles IV for Edward's French lands. Queen Isabella seized control of their son and refused to allow him to return to England, arranged the boy's marriage to the count of Hainault's daughter and used the dowry to pay for ships and mercenaries, and invaded England in September 1326. This, of course, led to Edward II's forced abdication in favour of his son in January 1327.
Many writers on this subject have assumed that Edward's sending his son to France is proof that that he was stupid, that he sent his son blindly unaware of the dangers, and that he fell into the cunning trap his wife and her ally/lover Roger Mortimer had planned for him. Edward's behaviour in August and September 1325 in fact demonstrates that he was completely aware of the risks, as I will prove.
Edward was meant to perform homage for Gascony and Ponthieu at Beauvais on 29 August 1325 to his brother-in-law Charles IV, his overlord for these lands. If he failed to do so, the lands would be forfeit to the French king. Edward's great-great-grandmother Eleanor of Aquitaine had brought the duchy of Aquitaine to the English Crown on her marriage to (the future) Henry II in 1152, while Ponthieu was Edward's inheritance from his mother Eleanor of Castile, countess of Ponthieu in her own right. He could not possibly allow these lands to pass under the king of France's control. On the other hand, there were several reasons why Edward was most reluctant to travel to France at this time. He was worried that he would be indicted in the French court for the 1322 death of his cousin Thomas of Lancaster, Charles IV's uncle.  His enemies, Roger Mortimer and other English exiles, were living on the continent, and Edward was afraid of assassination or capture. Edward was at war with France - the War of Saint-Sardos - from August 1324 to June 1325, so his relations with his brother-in-law Charles IV were a long way from cordial. Not to mention, Edward's ineptitude and tyranny meant that there was seething discontent in England, and he might have been worried that the country would erupt into rebellion during his absence.
Edward had other concerns. He could not take his favourite Hugh Despenser with him, as Despenser was loathed in France for his piracy of 1321; it was said that if he set foot there, he would be arrested and tortured.  But leaving Despenser behind was risky, too: he and his father Despenser the Elder, earl of Winchester, were also widely loathed in England, and feared that they would be put to death as soon as they lost the king's protection. As the Vita Edwardi Secundi says, they "realised that in the absence of the king they would not know where to live safely." 
So Edward could not go to France with Despenser, he could not go to France without Despenser, and he could not avoid performing homage. His only other choice, other than losing his French inheritance, leaving his friend behind to be murdered or taking him along and risking him being tortured, was to make his son Edward of Windsor duke of Aquitaine and count of Ponthieu in his place, and send him to perform homage instead. This had been suggested by the French at the end of 1324 or beginning of 1325, but Edward's counsellors rejected the proposal "with one voice." They did consent to send Queen Isabella to negotiate with her brother on Edward's behalf, but were understandably unwilling to send the twelve-year-old heir to the throne to an enemy country until peace had been established. (quant al point del aler nostre dame la royne et de mons' son filz, touz de une voice desconseillerent laler mons' le fitz quant a ore et si la qe la pees feust meux tretee et accordee: Regarding the journey of our lady the queen and Monsieur her son, they [Edward's advisers] with one voice counselled against the journey of Monsieur the son at present and until peace should be better negotiated and agreed; dated c. 13 January 1325.) 
Henry III had in 1253 been equally reluctant to send his fourteen-year-old son, the future Edward I, to Castile, as suggested by Alfonso X, then inciting a rebellion in Gascony with a view to invading and taking over the duchy, in case Alfonso took Edward as a hostage. And Edward II knew in September 1325 that sending his son to France also had serious drawbacks: he would lose control of his French lands and their income, and far more dangerously, the king was well aware that that his enemies could seize his son and use him as a hostage, not to mention the more general risks of sending the future king of England to a hostile country with whom he had only recently established a shaky peace (Edward signed a truce with Charles IV on 13 June 1325).
So in September 1325, Edward II was in an impossible position; every course of action open to him had serious risks and drawbacks. For this, of course, he only had himself to blame. If he had performed homage when Charles IV first invited him to do so, between Candlemas (2 February) and Easter (15 April) 1324, if he hadn't made himself so wildly unpopular, if he hadn't allowed the Despensers to make themselves so loathed, if he hadn't behaved so vindictively in 1322 and ensured that he and the Despensers had numerous enemies both at home and abroad, if he hadn't alienated Queen Isabella, and so on, he wouldn't have been in such a mess in the first place.
Edward's indecisiveness as to the correct course of action is painfully apparent. He spent the second half of August and early September 1325 hovering uncertainly in Kent prior to departure to perform homage, staying at Sturry, Langdon and Dover. Although some writers would have you believe that he was utterly and stupidly oblivious to the dangers of sending his son to France, this is in fact very far from being the case. Edward at first decided to go himself, then changed his mind, then decided to go himself, then changed his mind:
- Pope John XXII had heard by late June 1325 that Edward himself was going to France. 
- On 20 July, Edward appointed keepers of the truce between himself and Robert Bruce and ordered men to guard the coast of Northumberland during his absence overseas. 
- On 29 July, Edward told Robert Kendale, constable of Dover Castle, that he was going to France around the Assumption (15 August) "upon great and arduous affairs touching him and his realm," and ordered Kendale to provide as many ships as necessary for himself and the magnates accompanying him. 
- On 21 August, eight days before he was due to pay homage, Edward began issuing letters of protection for the retinue accompanying him to France. On the same day, he asked the Dominicans of Lincoln to pray for him, Isabella, Edward of Windsor, and their other children. 
- his Italian bankers the Bardi gave him over £3515 for his expenses, and silver plate worth £1768 to hand out as gifts at the French court. 
- on 24 August, Edward changed his mind about going, and told Charles IV that he had suddenly been taken ill and would not be able to come to France. This was almost certainly feigned; Edward was a healthy, strong and fit man who rarely suffered from illness. 
- on 30 August, the day after he should have performed homage at Beauvais, Edward changed his mind again and appointed his son regent of England while he went to France. 
-on 1 September, Edward told Louis Beaumont, bishop of Durham, that he was "shortly going to France" and had appointed his son as regent, and ordered Beaumont to keep the bishopric safe during his absence. 
- also on 1 September, Edward appointed Anthony Lucy and the earl of Arundel as wardens of Cumberland and Westmorland, and of the Welsh marches, during his absence overseas. 
- on 2 September, Edward changed his mind again and made his son count of Ponthieu. 
- evidently still unsure whether he was doing the right thing, Edward continued issuing letters of protection for the men accompanying him to France - him, not his son - on 3 and 4 September. 
- Edward began issuing letters of protection for the men accompanying his son on 5 September, but waited until 10 September before making young Edward duke of Aquitaine prior to sending him to the king of France.  The delay is a further indication of his uncertainty as to whether he was doing the right thing. But this remained his final decision, and the boy sailed from Dover on 12 September and performed homage to his uncle Charles IV at Vincennes on 24 September. 
- according to the chronicle of Adam Murimuth, who should know, because he was there, Edward and his advisers continued to discuss while he was staying at Langdon whether he should travel overseas. Edward was at Langdon from 24 August to 3 September. 
If Edward had been as oblivious to the consequences of his actions as many commentators have assumed and stated as fact, he would have blithely sent his son to France without a second thought, but as is apparent from his frequent changes of mind, he was most emphatically not oblivious; he was torn between the dangers of sending his son to France and going himself. He was also perfectly well aware of the dangers of sending his son unmarried, as proved by his injunctions to the boy both prior to his departure and in subsequent letters not to marry "without the king's consent and command."  Edward did not fall into Isabella and Roger Mortimer's cunningly-laid trap, and whether Isabella had ever even been in contact with Mortimer and his allies on the continent, men who had fled England after the battle of Boroughbridge in March 1322, is a matter for speculation. It is a possibility, but there is nothing to prove it.
In the end, it was Hugh Despenser who persuaded Edward not to go, and the Anonimalle says that the favourite "made a great sorrow, and lamented piteously to the king that if he passed beyond sea, he [Despenser] would be put to death in his absence," a story confirmed by Adam Murimuth and the Vita.  Edward must surely have remembered what had happened when he left Piers Gaveston at Scarborough Castle on 10 May 1312: his beloved was killed six weeks later.
And therefore, Edward made the decision for which he has unfairly been condemned as a stupid, blind fool ever since, and sent Edward of Windsor to France. He never saw his son again. (At least, not officially; unless the William the Welshman who met Edward III in 1338 was Edward II himself, but that's another story.)
In sending his son to France, Edward made a very bad decision, but that does not mean he made it blindly and unwittingly, and if he had gone to France himself and been assassinated or kidnapped, historians would no doubt ask how he could have been so stupid as to travel abroad himself when he could have sent his son instead. If neither he nor his son had paid homage and let Gascony and Ponthieu fall forfeit to the French Crown, he would of course be castigated for losing such an important part of his inheritance. If he had left Despenser behind, he would be sneered at for not caring if his friend was murdered. Hindsight is a wonderful thing, and it seems to me that whatever decision Edward II made in September 1325 would, in retrospect, have been the wrong one.
1) Mark Buck, Politics, Finance and the Church in the Reign of Edward II: Walter Stapeldon, Treasurer of England, p. 156, note 199.
2) Vita Edwardi Secundi, ed. N. Denholm-Young, p. 142.
3) Vita, p. 140.
4) Pierre Chaplais, ed., The War of Saint-Sardos (1323-1325): Gascon Correspondence and Diplomatic Documents, pp. 195-196.
5) Calendar of Papal Letters 1305-1342, p. 466.
6) Foedera, II, i, p. 603.
7) Calendar of Close Rolls 1323-1327, p. 496.
8) Calendar of Patent Rolls 1324-1327, pp. 161-2, 166-8; Cal Close Rolls 1323-1327, p. 503.
9) Natalie Fryde, The Tyranny and Fall of Edward II 1321-1326, p. 96.
10) Foedera, p. 606.
11) Cal Pat Rolls 1324-1327, p. 171.
12) Cal Close Rolls 1323-1327, p. 399.
13) Cal Pat Rolls 1324-1327, p. 171.
14) Cal Pat Rolls 1324-1327, pp. 173-175.
15) Cal Pat Rolls 1324-1327, pp. 167, 169-170.
16) Cal Pat Rolls 1324-1327, pp. 168, 173-175.
17) Chaplais, War of Saint-Sardos, p. 243; Cal Pat Rolls 1324-1327, p. 175; Cal Close Rolls 1323-1327, p. 507.
18) Adae Murimuth Continuatio Chronicarum, ed. E. Maunde Thompson, p. 44.
19) Cal Close Rolls 1323-1327, pp. 577-578.
20) The Anonimalle Chronicle 1307-41, from Brotherton Collection MS 29, ed. W. R. Childs and J. Taylor, p. 120; Murimuth, p. 44; Vita, p. 138.