You are right about defense being the one priority. The problem is that much that is called defense, such as "nation building", "global leadership", "commitments" and "power projection" has less to do with the Constitutional defense mandate and more with the current bipartisan elites' consensus that has its origins with Woodrow Wilson and FDR rather than with George Washington and the Founders.
The issue with "power projection" is that if we don't, the Russians and the Chinese will. Technology has made the world a lot smaller than when the founders envisioned our country. The US was well isolated from the rest of the world until WWII. After that, and especially with the advent of ICBMs, we are no longer so isolated. Also, I would rather fight the battle abroad than in our own backyard.