You are the Emperor of Britany and want to invade France but don't know why ?
I'll tell you how to do it.
1.Invite a claimant of France to your court. ( strong claim is prefered )
2.Grant him a county and a small kingdom ( like wales, even better if you grant him a viceroy title, this way he doesn't become too powerful )
3.Press his claim on the kingdom of france. ( you can press his claim right away if he has strong claim )
win the war, now the king of france is you vassal.
I'll tell you how to do it.
1.Invite a claimant of France to your court. ( strong claim is prefered )
2.Grant him a county and a small kingdom ( like wales, even better if you grant him a viceroy title, this way he doesn't become too powerful )
3.Press his claim on the kingdom of france. ( you can press his claim right away if he has strong claim )
win the war, now the king of france is you vassal.