It provides the US military with invaluable combat experience and allows it to test/improve its strategies, equipment, and intelligence capabilities.
(Of course, fighting wars so one can remain combat-ready is the very definition of treating other people as means to an end rather than as ends in themselves, and so very much offends against the moral sensibility. But the question here is about self-interest.)
(Of course, fighting wars so one can remain combat-ready is the very definition of treating other people as means to an end rather than as ends in themselves, and so very much offends against the moral sensibility. But the question here is about self-interest.)