A more thorough explanation:

Definition: Youngstown Sheet & Tube v. Sawyer, 342 U.S. 579 (1952) was a case in which the U.S. Supreme Court had to decide on the applicability of the President's national security powers on seizing private property. President Truman had ordered the Secretary of Commerce to take possession of and operate the mills in order to maintain steel production during the Korean War. The Supreme Court held that President Truman lacked either constitutional or statutory authority to seize the nation's strike-bound steel mills (the Court noted, however, that Congress would have had constitutional authority to do so).

Example: During the Korean War, a labor dispute arose between steel companies and their employees. As a result of the parties' failure to reach a collective bargaining agreement, the workers went on strike which paused the production of then much-needed steel for war materials. Fearing that such a stoppage would pose a threat to national security, President Harry S. Truman issued an executive order directing the Secretary of Commerce, Charles Sawyer, to seize the steel mills to keep them running. The steel companies brought action alleging that the seizure was unconstitutional.

Explanation: The example illustrates how President Truman used his national security powers to seize private property during the Korean War. However, the Supreme Court held that the President lacked the constitutional or statutory authority to do so, as the action was not explicitly authorized by an act of Congress, nor could it be reasonably construed as carrying out one of the authorities granted to the President. The Court noted that Congress would have had constitutional authority to seize the steel mills to keep them running during the war.

Justice Jackson's Concurrence: Justice Jackson's concurrence in this case also provided a tripartite scheme that courts would later apply to determine the constitutionality of presidential power. Namely, Justice Jackson explained that the President's power is greatest when the sought action is expressly authorized by Congress. Second, when Congress is silent on whether a certain presidential action is authorized, the President must rely on his power as the executive. Lastly, when the President takes action that goes against congressional authority, his power "is at its lowest ebb."