Which cabinet department is the president's "right arm in foreign affairs"?

The State Department, more specifically the Secretary of State, are considered the President's "right arm" in foreign affairs. The Secretary of State is a cabinet position that is in charge of dealing with foreign relations. This position and department can work with other countries on everything from economic policies to peace treaties. This department is extremely important in today's society, considering how often the US works with other countries for importing and exporting goods/services.