Final answer:
The President and Congress are assisted in conducting foreign policy by various governmental agencies, including the State Department, Defense Department, Department of Homeland Security, and intelligence agencies. The State Department is considered the most important agency for foreign policy due to its primary role in developing and implementing the President's foreign policy.
Step-by-step explanation:
In the United States, foreign policy is made and carried out by the executive branch, particularly the president, with the national security adviser, the State Department, the Defense Department, the Department of Homeland Security, and the intelligence agencies.
The National Security Act of 1947 and recent bureaucratic reorganization after 9/11 reshaped the structure of foreign policymaking.
Therefore, the governmental agencies that assist the President and Congress to conduct foreign policy include the State Department, Defense Department, Department of Homeland Security, and the intelligence agencies.
In my opinion, the State Department is the most important agency when it comes to conducting foreign policy because it is the lead U.S. foreign affairs agency and plays a primary role in developing and implementing the President's foreign policy.