The relation bar (−1…+1) is the gateway to everything: trade partnership, treaty signing, investment permission, tourist flow… Below −0.3 means embargo. Sending envoys warms relations step by step; war and ultimatums chill them.
Open the treaty window from the country card: if the relation threshold is met, it is signed. A treaty warms relations daily; it is protected for 60 days after signing, and later breaks only if relations fall clearly below the threshold. A defense pact can also drag you into war — sign carefully.
Your diplomats tour country after country month by month, warming up cold relations and signing the treaties that already clear their threshold, so you don't have to. Buildings like embassies set the headcount, your budget sets how many are actually in the field.
You lock in another country's surplus at a fixed price for years, and the delivery happens before ordinary trade. The price is negotiated: lowball it and they refuse, and you can't come back with a new offer for a while.
First you need an Investment Rights Treaty (they accept once relations are good enough). Then use Buy Factory on the country card: the facility stays where it is, and 70% of its profit flows to you. The list is sorted by payback time; if a facility earns nothing right now, the game warns you before you buy it.
Conquest, puppets and bulk buyouts accumulate heat. As heat rises the world cools toward you; cold countries form a coalition and start a strategic embargo, and if heat climbs further they may even declare war together. Slow down — heat fades over time and the relation damage is rolled back.