"We the People of the United States, in order to......" - That's how the Constitution begins. You know the rest, right? If you don't, try reading the Constitution. Then get back to me on what kind of constitutional amendment would be needed to provide for the common defense, e.g. maybe have a Constitutionally mandated Commander in Chief, maybe a Congress with the power to raise armies and navies, a militia.
I think you are too ignorant of our Constitution to discuss the issues.