The oath the president takes when assuming the office is prescribed in the United States Constitution, Article II, Section I
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.
George W. Bush took this oath on January 20, 2001. The oath is defend the Constitution, not the people, the territory or the property of the United States.
The President is the Commander in Chief, but defense is an obligation in the hands of the people, and not the first. From the Preamble:
We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.