How to use Leaflet to create interactive map which allows user to select country or territory and produce a popup with text on it?

My goal is to create an interactive map where its user can click or tap anywhere within a country or territory’s legally recognised borders, causing a pop up to appear. A big enough red circle will appear for countries/ territories which are too small to select. I have no idea on how to accomplish this.

Anyway, I created one, with pins instead and it looks awful.

Please give me advice on this.