[oldleft]

They did not start an army from scratch. Many of the Confederate officers went to West Point. The Confederate Army mostly US Army units that changed flags. They had already been trained and armed by the US.

[central_va]

They had already been trained and armed by the US.

Materially, they started from scratch.

[Michael Zak]

No U.S. Army unit switched over to the Confederacy.

And, thogh many southern officers went over to the rebels, hardly any southern enlisted men did so.