Trade is how countries cooperate with one another through the exchange of production. Trade helps all civilizations gain access to goods and services that are not produced domestically.
Nations can specialize in what has the advantage of production and import what they can not produce or produce inefficiently. In contrast, the nation can export the good it produces. This is how commerce works.
Trade negotiations take place through agreements between countries and through contracts between traders and buyers. These are ways to give security to both sides, ensuring a trade-prone environment.