Here's why:
* Ally: A country that is formally united with another for a common purpose, especially in war. This implies a strong level of trust and cooperation.
* Friendly: While this term suggests positive relations, it's not as specific as "ally". Countries can be friendly without being formal allies.
* Cooperative: This term indicates working together on specific issues, but doesn't necessarily imply a broader, long-term relationship.
* Partner: This term can be used in a variety of contexts, and doesn't automatically imply a friendly relationship.
Therefore, ally is the most appropriate term to describe a country with friendly relations with another.