The President of the United States
The President is the Head of the Executive Branch and generally viewed as the head of the U.S. Government. While he does have significant power, his power is limited by the Constitution. Specifically, the Constitution assigns the following powers to the President:
For convenience, we have divided these main powers into three categories: Head of State, Administrative, and Legislative Powers.
As Head of State, the President meets with the leaders of other countries. He has the power to recognize those lands as official countries and to make treaties with them. However, the Senate must approve any treaty before it becomes official. The President also has the power to appoint ambassadors to other countries, with the Senate's approval.
The President is also the official head of the U.S. military. As Commander in Chief, he can authorize the use of troops overseas without declaring war. To declare war officially, though, he must get the approval of the Congress.
The President's administrative duties include appointing the heads of each Executive Branch department. Of course, these appointments are subject to the approval of the Senate. The President also has the power to request the written opinion of the head of each Executive Branch department, regarding any subject relating to their department.