William, the duke of Normandy: 1048 to 1060
This is a bigger project than you can safely assume. It doesn't matter much how long it takes: because you are the aggressor, Bretons (from now on) will always consider you an enemy. When (or if) the time comes for you to raise an army of invasion to press your claim to the throne of England, Brittany will not be giving you its support: your cause in the future depends on your keeping Breton friendly feelings. This is not the way to get that. History will remember you as a trouble-maker of no more than average accomplishments.