Actually, you are correct. There is no provision within the Constitution for an army - for a Navy, yes.
However, within our nation's last hundred years or so, it became apparent we needed such. So, we need a Constitutional Amendment to change that.
In this day of hi-tech weaponry and smaller world, the US cannot rely on a civilian Militia to counter our enemies, as much as some dream we should.
Talk of a standing army in the US comporting with the Constitution for possible defense from forces without the confines is not the problem. The purpose of Obama’s CDF is not for combating threats from without - it is for oppressing dissent from within.