[Non-Sequitur]

Lincoln could and should have ordered his army to quit the garrison and move to a location within the border of the United States.

Why?

[PretzeLogic]

Lincoln could and should have ordered his army to quit the garrison and move to a location within the border of the United States.

Why?

Because it was the right thing to do. Forcing allegiance to a single individual or to a state is tyranny. On a smaller scale, it is called slavery.