What is the term for a government formally cooperating with another for a special purpose?

A coalition is a pact or treaty among individuals or groups, during which they cooperate in joint action, each in their own self-interest, joining forces together for a common cause. This alliance may be temporary or a matter of convenience. A coalition thus differs from a more formal covenant. Possibly described as a joining of 'factions', usually those with overlapping interests rather than opposing.