America's "defense" budget is really more than $1.2 trillion, more than double the Pentagon's official "base budget," once we include all the other related spending like that on "homeland security" and veterans affairs.
Disagreements between libertarians and realists are a problem. But in the current interventionist-dominated environment, the disagreements don't strike me as an especially big problem — at least for now.
Those who are advocating for new interventions in Syria and Venezuela show little interest in confronting the real costs of intervention. They just want to say they "did something" even if those things will turn out to be disastrous.