A war between two US allies is a matter of
foreign policy
Foreign policy has to do with US relationships with foreign nations. Two nations that are both US allies going to war with each other would cause something of a predicament for US foreign policy, determining how to respond in that situation.
The opposite of foreign policy is domestic policy. Domestic policy has to do with what goes on inside one's own country, whereas foreign policy has to do with what happens outside one's own country with other nations.