Age of Empires II: Definitive Edition

Playing on Standard difficulty

To get West Francia (purple) to attack you, all your 3 duches (allies) must be rebelling towards you. Their loyalty decrease with time and once it reaches zero, they rebel and start attacking you.
They become ally again after you defeat a certain number of their troops.

Get your economy booming, build several castles around your base for defense, start building an army of trebs and knights. For me, 5 castles were enough as a defense to stop attacks from the rebels (they don't send many units on standard).
Try not to kill too many of the rebelling units until all of them are rebelling towards you: you need the 3 rebelling at the same time for West Francia to become enemy, and if you kill too many of them, they'll become ally again.

It's time for you to attack West Francia. Now you can be ally with the rebels again ; West Francia will remain enemy until you defeat them anyway.
Note that you don't actually have to destroy the entire West Francia faction ; they will surrender after you've done enough damage to their city and units. It shouldn't be an issue with a heavy army of knights and trebs to take down castles and forums.

PS: As noted by Lord Kingslay3r in the comment, Bavaria takes a looong time rebelling on Standard, because they have a really slow economy. In my game they were stuck at 1 point for so long I thought my game glitched, but they eventually rebelled. So no worries !