I disagree.
The Constitution is not so much about what the government should or should not do. It expresses few ideals or goals, with the obvious exception of Bill of Rights.
The Constitution, as I’m sure you know, vests the war-making power jointly in the President and Congress. It says nothing about what the goals of a war conducted under it should be. It nowhere says nation-building is prohibited. The war-making powers, except for some irrelevant quibbles about how long an army budget can be for and suchlike, is essentially unlimited.
I agree in general that nation-building is not a good idea. But conservatives should be the last people to claim that anything they disagree with is therefore “unconstitutional.” That is a good part of how we wound up in this mess, with judges “finding” new interpretations of the Constitution to suit their druthers.
Many, many disagreeable things are not unconstitutional.
True, the argument is weak. I was also thinking “common defense” not “offense”, but I recognize that’s not an enumerated power. I’m sure there is an original intent argument. But they won’t change people’s minds either.
The best argument for non-interventionism: we’re broke.