From all enemies, foreign and domestic.
The Constitution is a list proscribed list of things the .gov must do, and how it must go about business. The Constitution, specifically the Bill of Rights, is a list of what some call "negative liberties" or things that the government cannot do. The term "negative liberties" is really a bad term, as the Bill of Rights just codifies natural rights, things you have simply by being your own person.
But how does that "protect and defend the Constitution" play into the Oath of Enlistment or Oath of Office?
Do we task the .mil with determining the Constitutionality of laws? No, that would be the judicial branch of government.
Do we task the .mil with writing laws and enforcing them? No, that would be the Legislative branch of government?
The .mil falls under the "Executive" branch, and the role of the Executive is really very narrow when you look at it from a Constitutional perspective. Instead of a "Priest King" responsible for the economy and leading the "free world" you have a Head of State responsible for defense and representing the US internationally (usually through the State Department).
So what does it really mean to "Protect and Defend" the Constitution? Does that give the .mil a right to round up secessionists? The Constitution denies any State the privilege of secession, and one could make the argument that the 10th Amendment makes that denial apply to the citizenry as well.
For an oath to have meaning, it must actually mean something. This nebulous idea of "defending the Constitution" is a whole lot different than "fighting Tyranny." As we have seen with Lincoln, FDR, GWBush and Obama, tyranny can be deemed Constitutional. From rounding up Americans and putting them in concentration camps, confiscating privately owned gold, forcing travelers to pose nude for bureaucrats with no power to arrest, to setting up domestic roadblocks and interfering with the Supreme Court upheld right to freely travel....all Constitutional acts according to the "for the general welfare" language, become Unconstitutional?
I think people do not understand exactly how much weasel wording the Founding Fathers put into the Constitution that allows a veneer of "rule of law" on a Congress unconstrained by conscience. Because that is the crux of the Constitution, the most powerful branch of .gov is the Legislature, and the power to change the Constitution, the power of the purse strings, all lies with the Legislature. I can only assume that the Founding Fathers assumed that the Legislature would always be composed of the best of us, the most educated, the most erudite and eloquent, the most insightful and thoughtful.
So what does the oath to protect and defend the Constitution really mean? It obviously means different things to different people who have raised their right hand and given their solemn affirmation. But at this point the oath is functionally meaningless, as the encroaching tyranny still wears the mantle of "Constitutionality." I guess if you are a Judge, required to pass judgement on what passes muster and what doesn't, that oath means much more. For LT Beltbuckle or PVT Tentpeg, the oath has no real bearing on operations.