Edmund, Duke of Somerset (or Edmund, Marquess of Dorset as he was at the time) replaced the Duke of York as lieutenant-general and governor of France on 24 December 1446. The court party, dominated by Edmund's aged uncle, Cardinal Beaufort and the Duke of Suffolk, probably thought that York was too committed to the war. Their policy, by this time, was peace on almost any terms. King Henry VI's marriage to Margaret of Anjou (1445) had been undertaken on the basis of a truce with a secret agreement to cede Anjou and Maine.
Obviously, this secret could not be kept for ever, but the surrender was opposed by York, the Lancastrian establishment in Normandy (who could see the strategic implications) and above all by the King's uncle, Humphrey, Duke of Gloucester. However Duke Humphrey had little influence now and in early 1447 was arrested and shortly after died. Many people believed he had been murdered, but there's no particular proof of this. He may simply have had a heart attack or the like. Meanwhile York was given a 10 year contract to govern Ireland. This was not necessarily demotion or banishment as such, but York was undoubtedly aggrieved, especially as the Crown's debt to him amounted to tens of thousands of pounds.
The appointment of Beaufort to the French post was not, however, all that ridiculous. He had a mixed track record as a soldier, admittedly, but he had had some success, while York, though competent, was not exactly Robert E. Lee. (It might be added that even Lee, Cromwell and Marlborough working together as a team might have struggled to keep the French out of Normandy for very much longer, given the appalling military situation and almost total lack of finances.)
Anyway, the agreed surrender of territory proceeded, despite the attempts of local commanders to be as awkward as possible so as to drag matters out. These stalling tactics made the French wonder about English good faith. If they were sufficiently perceptive, they probably also realised that Henry VI's government was somewhat lacking in grip.
Somerset (as he became in March 1448) was not especially tactful in his dealings with Charles VII, indeed he was rather discourteous, and this cannot have helped in so delicate a situation. Negotiations to resolve the situation were about to begin when an English force seized the town of Fougeres, on the borders of Brittany. Naturally this alienated the Duke of Brittany more than somewhat and gave the French justification for believing the truce had been broken.
Meanwhile, having allowed the capture and sack of Fougeres, the English did not give assistance to the mercenary captain involved and he was eventually forced to capitulate. At the same time Somerset's negotiations - or perhaps the word is dealings - with Charles VII failed miserably, since the French King, quite reasonably, had no faith in Somerset's honesty.
In the Spring of 1449 hostilities began in earnest. It's tedious to recite the tale of towns falling, one by one, and the process speeded still further after Charles VII declared formal war on 31 July. Rouen was captured on 29 October. Somerset obtained a safe conduct to England for his family and himself, and for many of his supporting cast, including Shrewsbury, Abergavenny and Roos. In return he had to agree to surrender not only Rouen but several other fortresses, pay a hefty ransom, and leave hostages behind to secure his good faith. By August 1450 the remaining Lancastrian holdings in northern France (except Calais) had fallen, and the last (rather feeble) English field army defeated.
It was a disaster, and there were many (notably York of course) who put much of the blame on Somerset. However, with Suffolk's fall from power, and subsequent murder in May 1450, it was Somerset who had Henry VI's confidence and became dominant at court.
More another day...