When turning on Messages in iCloud, how does it treat messages that were previously deleted on one device, but not another?

I have long put off turning on Messages in iCloud because I have deleted messages on one device knowing they are still on my other device. If I turn on Messages in iCloud, will those old messages be deleted from the devices I wanted to keep them on? Or will the old deleted messages reappear on the devices I wanted them deleted from? How does the sync determine which messages to keep and which to delete when different devices have retained different messages?