The checks and balances system in place among the branches of the U. S. government acts to prevent one branch from having too much power. For example, Congress limits the president's power by its right to deny a presidential appointment to a federal court or other position. The president limits the power of Congress by his ability to veto legislation, and the Supreme Court limits the power of the other two branches by its ability to declare an action unconstitutional.
The nation's founders felt it so important to keep each branch in check that they wrote the system of checks and balances into the Constitution. Having lived under the rule of the British monarch, the founders did not want any person or group ever to have the opportunity of gaining too much power.
Very important among the checks and balances that the forefathers established is the right of Congress to impeach, try and remove from office a president who does his job in an illegal or improper manner. As of 2015, Congress has used this power only twice: first with Andrew Jackson, and later with Bill Clinton. These presidents were impeached but not removed from office.
The president's duty to appoint Supreme Court justices gives him a measure of authority over the Court and what type of decisions it is likely to render. As stated, however, Congress must approve the appointment. Only Congress may declare war. Also, when a president vetoes legislation, Congress may override the veto.