This isn’t true. Firstly it isn’t against the Constitution to maintain a standing Army. What the Constitution says in Article I, Section 8, Clause 12 is “The Congress shall have Power To ...raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years...”

The people drafting the Constitution knew that a standing army could be abused by a tyrant, but having served in the Continental Army also knew how vital a standing Army was to maintain peace. That’s why they designed it so Congress controls the purse strings and authorises military spending only for 2 years at a time. The executive may give the orders, but there’s a time limit on the Army he can give orders to.

And the second part - the US has had a standing Army since 1796. You remember Robert E Lee resigning from the Army to join the Confederacy? If there was no standing Army, what did he resign from?

But even leaving aside these two historical facts, think about it logically. Throughout history military advantage has always been with the better trained, more experienced troops. Even if you rely on conscripts in a war, they need to be trained and led by professionals. Saying a standing army shouldn’t exist is like firing all your firefighters and saying you’ll start hiring when someone reports a fire.