A fundamental principle undergirding the design of American government is that of the separation of powers, which prescribes the parcelling out of the various powers and functions of government to separate and relatively independent levels and branches of the federal system in order to prevent their all being controlled at the same time by any potentially tyrannical political faction. But, to the way of thinking of the Framers of the Constitution, the long-term survival of free popular government would require more than simply a purely formalistic separation of governmental functions and powers into completely independent organizational jurisdictions. Ambitious and unscrupulous office holders in one or another of the various branches and levels of government could be expected to encroach upon the powers and authority of the other branches and levels from time to time, and this would gradually bring about a tyrranical concentration of powers unless the leaders in the other parts of the government could be given the necessary constitutional means and personal motives to resist the encroachments of the others. "Ambition must be made to counteract ambition. The interest of the man [the officeholder] must be connected with the constitutional rights of the place."
From this the Framers concluded there was a need for the Constitution to include a built-in set of "checks and balances" -- the necessary legal weaponry for each branch to defend itself against encroachments on its independence and authority by the others. In most cases, this contervailing power is purely negative, usually taking the form of some special constitutional grant of authority for one branch to say "no" to at least some of the specific decisions of the other branches in their own fields of specialization and then make it stick. (Some examples: The two houses of Congress may finally agree on a compromise to pass or repeal a law, but the President can veto it. President and Congress can agree on passing a law, but if the federal judiciary declares it to be unconstitutional the courts will refuse to treat the law as valid or enforceable. The courts can issue orders and injunctions for particular individuals to act or refrain from acting in particular ways, including public officials, but the power of the law enforcement agencies in the executive branch is needed to enforce them if the individuals in question decide to disobey. The Congress cannot control the way a judge will rule in a particular case before him, but Congress has the power to define and redefine the jurisdiction of the various federal courts. The President has general supervision of the conduct of foreign policy and military policy, but his treaties must be ratified by the Senate before they enter into force, and only Congress can appropriate public money to pay for such things as the raising of an army or the dispensing of foreign aid.) More...