>In time of actual war, great discretionary powers are constantly given to the Executive Magistrate. Constant apprehension of War, has the same tendency to render the head too large for the body. A standing military force, with an overgrown Executive will not long be safe companions to liberty. The means of defense against foreign danger have been always the instruments of tyranny at home. Among the Romans it was a standing maxim to excite a war, whenever a revolt was apprehended. Throughout all Europe, the armies kept up under the pretext of defending, have enslaved the people.
-James Madison, Speech at Constitutional Convention, 1787
>A politic minister will study to lull us into security, by granting us the full extent of our petitions. The warm sunshine of influence would melt down the virtue, which the violence of the storm rendered more firm and unyielding. In a state of tranquillity, wealth and luxury, our descendants would forget the arts of war, and the noble activity and zeal which made their ancestors invincible. Every art of corruption would be employed to loosen the bond of union which renders our assistance formidable. When the spirit of liberty which now animates our hearts and gives success to our arms is extinct, our numbers will accelerate our ruin, and render us easier victims to tyranny. Ye abandoned minions of an infatuated ministry, if peradventure any should yet remain among us!—remember that a Warren and Montgomery are numbered among the dead. Contemplate the mangled bodies of our countrymen, and then say, What should be the reward of such sacrifices ? Bid us and our posterity bow the knee, supplicate the friendship, and plough, and sow, and reap, to glut the avarice of the men who have let loose on us the dogs of war to riot in our blood, and hunt us from the face of the earth? If ye love wealth better than liberty, the tranquillity of servitude, than the animating contest of freedom—go from us in peace. We ask not your counsels or arms. Crouch down and lick the hands which feed you. May your chains set lightly upon you, and may posterity forget that ye were our countrymen.
-Samuel Adams, American Independence Speech, 1776
The executive Power shall be vested in a President of the United States of America
This executive power is the vehicle by which various acts of legislation which "provide for the common Defence and general Welfare of the United States" (I.8) take effect.
> I do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States.
Note the order: first responsibility is to execute the office, the second is to protect the Constitution. The responsibilities of the Office of the President are laid out in Article II of the Constitution, starting with Section 2. The very first one is: "The President shall be commander in chief of the Army and Navy of the United States, and of the militia of the several states, when called into the actual service of the United States[.]"
It's not unreasonable for him to say his most important job is to keep America safe. Indeed, the historical evidence shows that one reason the Constitution created a strong executive (where the Articles of Confederation had no executive), is because the failure of the government under the Articles to quell certain internal rebellions.