The Constitution divides war powers between the Congress and the President. This division was intended by the framers to ensure that wars would not be entered into easily: it takes two keys, not one, to start the engine of war.
Explanation:
The Constitution's division of powers leaves the President with some exclusive powers as Commander-in-Chief (such as decisions on the field of battle), Congress with certain other exclusive powers (such as the ability to declare war and appropriate dollars to support the war effort), and a sort of "twilight zone" of concurrent powers. In the zone of concurrent powers, the Congress might effectively limit presidential power, but in the absence of express congressional limitations the President is free to act. Although on paper it might appear that the powers of Congress with respect to war are more dominant, the reality is that Presidential power has been more important--in part due to the modern need for quick responses to foreign threats and in part due to the many-headed nature of Congress.