Let us begin with what the Constitution says -
1. The Congress shall have the Power... To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water; To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years; To provide and maintain a Navy; To make Rules for the Government and Regulation of the land and naval Forces; To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions; To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress.... U.S. Constitution, Art. I, sec. 8.
2. No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or is such imminent Danger as will not admit of delay. U.S. Constitution, Art. I, sec. 9.
3. The President shall be Commander in Chief of the Army and the Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States.... U.S. Constitution, Art. II, sec. 2.
That's it. That's everything the Constitution has to say about the military. Please note that it says not one word about the purpose of the military (it does speak briefly about the purpose of the Militia when called into service of the United States or in times of imminent danger of invasion). So the question arises - where do we look for the purpose of the military if not the Constitution?
I would submit that we look to the British in any case where the Constitution does not specify a variation from British meanings and understandings. I offer support for this statement as follows -
1. The Constitution says "The Privilege of the Writ of Habeas Corpus shall not be suspended...." U.S. Constitution, Art. I, sec. 9. The Constitution does not, however, say what the Writ of Habeas Corpus is. That is because it was well understood that it meant in the Constitution the same as it meant in British law.
2. The Constitution says "The Congress shall have Power... to grant Letters of Marque and Reprisal," U.S. Constitution, Art. I, sec. 8, but does not define letters of marque and reprisal. It was understood that the Constitution meant what British law meant by these terms.
3. The Constitution says "The judicial Power shall extend to all Cases, in Law and Equity, arising under this Constitution, the Laws of the United States, and Treaties made...." U.S. Constitution, Art. III, sec. 2. It does not define what law and equity mean. That's because law and equity were to mean what they meant in Britain.
4. Example of the opposite: when the Constitution intended terms to mean something other than they meant in Britain, it so specified, as in the definition of treason. "Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies...." U.S. Contitution, Art. III, sec 3. Here we see a specific definition of treason, because it was the intent of the Framers that treason should mean something different in the United States than it meant in Britain.
So far, we have two principles. First, the Constitution does not specify what the purpose of the military is. Second, where the Constitution does not specify something, that thing should be what the British understood it to be in 1787.
Next, does anybody seriously believe that the British considered the British Army and the Royal Navy to be for the purpose of defense only in 1787? I would suggest that the British considered the military to be a tool of the government, for defense, conquest, exploration, and whatever other task the government set it. I think millions of dead American Indians, Africans, Europeans, Indians, Chinese, Pacific Islanders, and Australian aborigines would agree.
Therefore - the purpose of the U.S. military is to do whatever the government tells it to do, as limited by the laws of war.
I invite anyone who disagrees to challenge this argument, or to cite authority to the contrary.
Have a nice day.