Answer:
The foreign policy of the United States is the result of a complex elaboration process, of great contradictory debates solved by arbitrations. On the other hand, the American constitutional system provides for an accurate distribution of powers (checks and balances), which gives the different actors (presidency, Congress, civil society, etc.) a power capable of influencing in a very different way the elaboration of the foreign policy of the United States. This observation differs from the commonly accepted opinion about the idea of a power gathered in the hands of a few, ending in a monolithic policy and without debate.
Certain foundations of the foreign policy of the United States are present since its creation. It is essential to know them to learn the whole decision process. On the other hand, decision-making is shared between the executive branch (the presidency and the administration) and the legislative branch (the Congress). Both suffer the influence of civil society (pressure groups, electorates, think tanks, etc.).
The president negotiates treaties with foreign nations. He is also the Commander-in-Chief of the armed forces, and as such has broad authority over them, once they are deployed. The Secretary of State is the Minister of Foreign Affairs of the United States and is the primary conductor of international diplomacy.
Congress has the power to declare war, but the president has the ability to send military troops to an area for 60 days without the approval of Congress, although in all cases they have granted it later. The Senate (one of the two chambers of Congress) also has the exclusive right to approve the treaties made by the president. The Congress is likewise responsible for approving the bills that determine the general character of the foreign policy of the United States.
The third arm of the government is the Supreme Court, which has traditionally played a minimal role in the country's foreign policy.