I think most people address encroachments with actions for trespass.
If you “sleep on your rights”, meaning that you kick people off your land, you start to lose the right to kick them off ever. Generally, it’s 20 years to lose title, but much less to lose the right to stop them from using your land. Easements are complicated, but if you do not object punctually, (within five years, usually), they might acquire the permanent right to use your land.
It’s probably not enough to just kick and scream. I think you might need to evict them to keep the land.
Also, if they start improving your land and you do not object, that’s really really bad. If you do not object punctually, they’ll argue you gave them tacit permission which –because it induced foreseeable detrimental reliance– can no longer be withdrawn.
You should probably at least consult an attorney to find out what you have to do to keep from losing the land at civil law. Knowing the civil remedy will probably get you a better negotiated settlement.
Your property line on the deed might not be where your fences are. You should probably check that and see if you are listed as the owner of the disputed land. If you aren’t, you or your previous owner probably adversely possessed it years ago. Once again, you’ll need a lawyer for local laws, etc.