While I think there is waste that could be removed from the Pentagon budget, this is the one area that should be fully funded before all of the non-Constitutional “entitlement” programs. If we have insufficient ability to “provide for the common defense”, we have no country.
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.