The War Powers Act, officially named the Joint Resolution Concerning the War Powers of Congress and the President, was enacted in 1973 by the 93rd Congress of the United States. (A joint resolution is one passed by both houses of Congress: the Senate and the House of Representatives.)

This federal law was passed in order to prevent the overreach of the President in committing troops to armed conflict outside the United States without the approval of Congress. This law stipulates that U.S. Armed Forces can only be sent into action abroad if there is an actual declaration of war by Congress, i.e."statutory authorization," or in case of "a national emergency created by attack upon the United States, its territories or possessions, or its armed forces."

The Senate Foreign Relations Commission stated that the purpose of this law is to prevent clandestine and unauthorized military support activities, and "to prevent a repetition of many of the most controversial and regrettable actions in Indochina." (The United States fought the Korean and Vietnam wars without actual declarations of war.)

President Nixon contended that the legislative veto provision of this law, permitting Congress to direct the withdrawal of troops by concurrent resolution, was unconstitutional. He further contended that the provision requiring withdrawal of troops after 60-90 days unless Congress passed legislation authorizing such use was unconstitutional since it checked presidential powers without affirmative congressional action.
In fact, every President since the enactment of the War Powers Resolution has asserted that the law is an unconstitutional infringement on the President’s authority as Commander-in-Chief. For instance, President Clinton violated the law in 1999 during the bombing campaign in Kosovo that extended past the 60-day limit. And, while President George W. Bush requested and received a resolution of support for the Iraq War from Congress, Bush stated that his compliance with the War Powers Act were not meant to imply that he acknowledged the act's constitutionality. Further, in 2011 President Obama was accused of violating this law for continuing military engagement in Libya.)

Although the War Powers Act was intended to provide a way to control an overreach of the president as commander-in-chief, demanding that he confirm with Congress prior to military engagements, it has not been entirely effective. Nevertheless, it does act as part of the checks and balances of the branches of government.

The War Powers Resolution (or Act) was a resolution that was passed by Congress in 1973.  The point of the resolution was to place a limit on the power of the president to commit troops to war zones.  It is an example of checks and balances because it is an attempt by Congress to exert some power over the president and prevent the president from having complete freedom of action.

During the Vietnam War, Congress came to feel that it did not have enough power over the military.  The Constitution says only Congress can declare war, but presidents had sent troops to Vietnam without asking Congress to declare war.  That way, Congress really had no say on the matter.  To attempt to rectify this, they passed the War Powers Act.  The Act said that the president has to get permission from Congress to keep troops in a war zone longer than 90 days.

The Constitution sets up a system of checks and balances in which each branch has the ability to limit what the other branches can do.  The War Powers Act attempts to limit what the president can do in terms of committing troops to war zones.  Therefore, it is an example of checks and balances.