What is the most important check of the president in foreign policy?
Perhaps the most important check on the president in foreign policy is that fact that the power to declare war is invested in the Congress. Not only is Congress constitutionally invested with this power, but it has the power to restrict funding for military actions as well. Since World War II, however, presidents have regularly claimed the right to conduct limited military engagements under their powers as Commander in Chief. The "police action" in Korea as well as the military buildup in Vietnam were both examples of this practice.
In 1973, with the conflict in Vietnam winding down, Congress passed the War Powers Act, which placed limits on the power of the President to make war without the consent of the legislative branch. Specifically, it required the President to report to Congress within seventy-two hours to justify any military action. Since Vietnam, however, Congress has not offered much resistance to presidents who wished to make war, notably in the aftermath of 9/11, when it voted to grant the President the power to make war on any nation or organization involved in the terrorist attacks. This law was used as the basis for the war in Afghanistan, and a similar measure authorizing military force against Iraq passed Congress a little more than a year later.