In game, you can make new treaties with other nations. To create a new treaty, first open the Political Sphere window. Click the button "Treaties" and a new window that list all active treaties you are part of, a list of the treaties that are proposed to you,a box in which you can set filters for the lists, and two buttons at the bottom. One of the buttons should say "New Treaty". Click it, and a new window will open up. From the new window, you will be able to create your new treaty.