According to Adam Cohen in The New York Times:
Given how intent the president is on expanding his authority, it is startling to recall how the Constitution’s framers viewed presidential power. They were revolutionaries who detested kings, and their great concern when they established the United States was that they not accidentally create a kingdom. To guard against it, they sharply limited presidential authority, which Edmund Randolph, a Constitutional Convention delegate and the first attorney general, called “the foetus of monarchy.”
The founders were particularly wary of giving the president power over war. They were haunted by Europe’s history of conflicts started by self-aggrandizing kings. John Jay, the first chief justice of the United States, noted in Federalist No. 4 that “absolute monarchs will often make war when their nations are to get nothing by it, but for the purposes and objects merely personal.”
Many critics of the Iraq war are reluctant to suggest that President Bush went into it in anything but good faith. But James Madison, widely known as the father of the Constitution, might have been more skeptical. “In war, the honors and emoluments of office are to be multiplied; and it is the executive patronage under which they are to be enjoyed,” he warned. “It is in war, finally, that laurels are to be gathered; and it is the executive brow they are to encircle.”
When they drafted the Constitution, Madison and his colleagues wrote their skepticism into the text. In Britain, the king had the authority to declare war, and raise and support armies, among other war powers. The framers expressly rejected this model and gave these powers not to the president, but to Congress.