The civil war demonstrated that the Chief Executive's powers derive directly from the Constitution and have as its aim the preservation of the Union through Law which is binding on the States.

The check on those orders would be impeachment, ultimately.

Edited to add: https://constitution.congress.gov/browse/essay/artII-1/ALDE_00000243/

Further edited to also add: https://www.johnadamsacademy.org/apps/pages/index.jsp?uREC_ID=2003858&type=d&pREC_ID=2094472