Does the secretary of defense have the power the declare war?

Answer 1

No. In the United States, the Secretary of Defense is really an Administrative position, not an Operational position.

That is, the SecDef is in charge of running the Department of Defense from an administrative standpoint: making sure that procurement runs smoothly, congressional and presidential directives are carried out, and that the organization of the military is efficient. While legally the US Military chain of command runs through the SecDef to the President, the reality is that the SecDef would never make an operational decision without a directive from the President to do so.

Both the President and Congress have the power to make military policy, though Congress' power to do so is very limited.

Currently, wars are run by the President. He makes the final decisions on any actual operational action. However, for smooth and efficient working, most decision making is devolved (i.e. passed down to) the regional commander in charge of the area where combat is taking place. These regions are called Unified Combat Commands, where the leader in charge of such command directs the actions of all U.S. military assets (regardless of branch) in that area.

For instance, the War in Iraq falls under the direction of the Central Command. While the President maintains ultimate control (and, thus can change operational policy and make specific directions whenever desired), the day-to-day running of the war is done by the officer in charge of CentCom.

Answer 2

No. only congress has the power to declare war. He as well as the President can authorize military intervention, but only for a certain time period.