As with virtually all U.S. government policy decisions, the executive branch, including the president, and Congress share responsibility in what ideally is a collaboration on foreign policy issues.
Congress controls the purse strings, so it has significant influence over all kinds of federal issues --including foreign policy. Most important is the oversight role played by the Senate Foreign Relations Committee and the House Committee on Foreign Affairs.
The House and Senate Committees
The Senate Foreign Relations Committee has a special role to play because the Senate must approve all treaties and nominations to key foreign policy postings and make decisions about legislation in the foreign policy arena. An example is the usually intense questioning of a nominee to be secretary of state by the Senate Foreign Relations Committee. Members of that committee have a great deal of influence over how U.S. foreign policy is conducted and who represents the United States around the world.
The House Committee on Foreign Affairs has less authority, but it still plays an important role in passing the foreign affairs budget and in investigating how that money is used. Senate and House members often travel abroad on fact-finding missions to places deemed vital to U.S. national interests.
Certainly, the most important authority given to Congress overall is the power to declare war and to raise and support the armed forces. The authority is granted in Article 1, Section 8, Clause 11 of the U.S. Constitution.
But this congressional power as granted by the Constitution has always been a flashpoint of tension between the Congress and the president's constitutional role as commander-in-chief of the armed forces. It came to a boiling point in 1973, in the wake of the unrest and divisiveness caused by the Vietnam War, when Congress passed the controversial War Powers Act over the veto of President Richard Nixon to address situations where sending U.S. troops abroad could result in involving them in armed action and how the president could carry out military action while still keeping Congress in the loop.
Since the passage of the War Powers Act, presidents have viewed it as an unconstitutional infringement on their executive powers, reports the Law Library of Congress, and it has remained surrounded by controversy.
Congress, more than any other part of the federal government, is the place where special interests seek to have their issues addressed. And this creates a large lobbying and policy-crafting industry, much of which is focused on foreign affairs. Americans concerned about Cuba, agricultural imports, human rights, global climate change, immigration, among many other issues, seek out members of the House and Senate to influence legislation and budget decisions.