Alliances and treaties
Allies fight beside you, trade with you and open their airfields to you.
A military alliance means you stand up for each other. When one of you is attacked, the other may join the war, though not every ally keeps its word.
A trade alliance adds to your income and lets you run supply routes from your partner's fields.
A passage permit lets your forces cross a country in peace for a few days. A ceasefire stops a war for a while, and peace ends it.