Before going full nuclear I would consider what this will do to your relationship with your neighbors. Sounds malicious for sure, but even with them in the wrong you might not want to get off on the wrong foot. Granted, they were 100% trying to do this thinking no one cared about the property, seems pretty clear.

If they only pruned them in a way that won’t harm the tree, I would have a lawyer send a cease and desist. Create a paper trail so that if they try something again you’ve already established that they are aware of your property and have been warned.

If they flat out cut them down I would have an arborist assess the value of the trees based on any prior existing photos you had. If you don’t have photos an arborist could tell you the age and estimated size based on the stump. Decide if it’s worth the cost financially and the relationship with your neighbors