You want an RP reason?
Henry V was a warrior of great magnitude. He had launched an attack on the mad Charles VI of France, and his forces looked invincible. Within a year, Caux and Picardie were made English lands, effectively connecting the English strip of northern France, from Calais to the Seine river. At the meeting in Troyes, Henry pushed for his line of inheritance, claiming the throne of France. This decision was met with mixed feelings by the local nobles in Anjou and Normandy, and they felt threatened by endless warfare between Henry and the Dauphin, Charles. They had to choose sides, and in Gascogne and Normandie the Dauphin was the first choice, and the castles changed their flags. But in Caux and Picardie the English were the main candidates.
But then King Henry V died. His son Henry VI, only a child, was put on the throne. The administration of England chose the Duke of Gloucester to act as king until the minority of Henry VI had ended. The Duke was a man of economics, and he came to the wise conclusion that peace was preferred over war with the Dauphin. So he stepped away from the English claims on Paris and decided to concentrate on the rich Lowlands, where Burgundy were lords. Flandern was conquered, and several years later the English territory was extended to the whole strip of Holland and Zeeland to border Friesen.
Gloucester also focused very much on the Royal Navy, and it was enlarged a great deal during the 1420s. This new fleet was now powerful enough to support an invasion of Norway and Denmark, the contestants in the North Sea. Gloucester reckoned that by striking as soon as possible, the English could be lords of the North Sea. Also, there were rumours of the Vikings having knowledge of far off lands; lands Gloucester wanted to claim for England, far away from greedy Europeans.
But this invasion of Scandinavia could not be performed with an aggressive, newly crowned Charles VII up the English ass. Scotland had been vassalised, and Ireland was under control, but the Southern front was not. Charles and Gloucester had become good friends, though, and the two kingdoms signed a pact against Burgundy together, giving France the southern regions of Bourgogne, while England would complete the hegemony over the Dutch by annexing Artois. With this foothold on the continent, the Holy Roman Empire could be watched over, while England took care of the Seas, reaching where no man has ever reached. From the friendly Portuguese they had gotten information of great riches in far away lands, and Gloucester was eager to lay his hands on it. The Portuguese themselves had been treacherously attacked and weakened, however, so the Duke decided it had to be up to him and his admirals. The seas were open...
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The thing about RP is that you can have it at any level. Why not just change all our policies at every monarch change? If you want RP rules, please formulate it well and put it in the rules in a clear manner. The current rule set mentions nothing about having to follow the event choices that are made.