It is very hard for any nation, especially one as allegedly well-intentioned as the United States, to resist the messianic temptation, especially when we see the genuine political and social needs of the nations of the world.
But it is very important to remember that the Biblical fruits of covenant-keeping faithfulness to God cannot be exported without first exporting the source of those blessings — the preaching of the gospel of Christ and His life-transforming (and culture-transforming) work. This is the essential difference between 18-19th Century colonialism and the imperialist efforts of the super powers in the 20th Century. The former usually sent missionaries first, where the latter have sent the army.
Nor can we properly apply the Biblical maxim of "love your neighbor" directly to the state. The state, as a "minister" of God for the sustaining of righteousness and justice (cf. Rom. 13:1ff) is strictly limited in the scope of its legitimate use of coercion (the "power of the sword"). To give to one, it must first take from another, and God in His wisdom has restricted the circumstances under which the state can coerce its citizens into helping another nation.~ Roger Wagner, "Vietnam: Biblical Reflections on National Messianism"
I'm especially interested in hearing the biblical support for Wagner's last assertion. (I wonder if I might simply call and ask him?)