Generally speaking, the president, secretary of defense, and chairman of the joint chiefs of staff deal principally at the level of war.

All of them deal with war at the executive level. This means that they are responsible for commanding and deploying the army. The president of the United States is, by the power vested in him by the constitution, the commander-in-chief of the US military, which means that he is the highest ranking person in the military hierarchy. His orders are carried out by the lower ranking members and therefore he possess the executive power. He however does not have the power to declare war, which falls under the jurisdiction of the Congress.