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While you probably already know this, others may not: there is an organization called the Oath Keepers which is comprised of current and former uniformed members of military and civil services who affirm what you've stated - that the oath they took was first and foremost to defend the Constitution.

From their site (http://oathkeepers.org):

Oath Keepers is a non-partisan association of current and formerly serving military, reserves, National Guard, veterans, Peace Officers, and Fire Fighters who will fulfill the Oath we swore, with the support of like minded citizens who take an Oath to stand with us, to support and defend the Constitution against all enemies, foreign and domestic, so help us God. Our Oath is to the Constitution.