A treatise is a formal written paper about a specific subject. It’s like an essay but longer. A treatise is usually about a serious subject, so you might read a treatise on democracy, but you probably won't read a treatise about chewing gum.
The word treatise is related to the verb treat, in the sense of "deal with,” like the way a doctor treats a patient. Like a doctor, a treatise examines a subject from all angles. It has nothing to do with treats, unless you’re writing a treatise on desserts, which would be weird. More appropriate topics for a treatise include philosophy, religion, economics, and so forth. An example of a political treatise is The Prince by Machiavelli, which basically argues that “the ends justify the means.”