Our Constitution was not a proclamation written by a convention. It was a negotiated contract for uniting thirteen sovereign independent states. Nothing like that had ever been done voluntarily, and few nations have matched it in two hundred years, even with the use of force.
The Founding Fathers were in general heatedly opposed to the idea of a domineering monarchy, but frankly disappointed with the lack of energy displayed by the unicameral Continental Congress under the Articles of Confederation. They were more or less satisfied with the Judiciary and the Common Law, although there had been instances of rebellion against Governor-dominated colonial Judges. James Madison tells us in the Federalist Papers that he was greatly taken with Montesquieu's concept of the division and balancing of powers. Probably as a result of the prodding of Madison, the Constitutional Convention's favored system of readjusting this arrangement was to increase the number of centers of power, while setting these centers against each other in order to maintain a balance. A concept which gained popularity at the convention was to attempt to give these centers the power to defend themselves, presumably against other centers competing for the same power. The President thus had veto power over acts of Congress, but Congress had partial power to override the veto. Congress could declare war, but the President was Commander in Chief. The President was given the power to execute the laws, but Congress controlled his purse. The Federal Government had control over international and interstate affairs, but the states had control of almost everything else. Although it took a few years for Chief Justice John Marshall to assert it, the U.S. Supreme Court had the right to declare acts of Congress (and the President) to be invalid because unconstitutional, and the Supreme Court had the underlying right to declare what the Constitution meant. On the other hand, the President appointed the federal Judges, with the consent of the Senate. The Chief Justice conducted impeachment proceedings against the President, the Congress had to agree with the decisions. In subsequent centuries, many little tweaks have been added to this system of governmental components holding each other in check, because it takes time for imbalances of power to be discovered and exploited. The general concept was to encourage centers of power to expand and innovate, while counting on internal jealousies to hold this "energy" in check.
It's often difficult to guess in advance whether a check and balance system is truly balanced, but over time it is possible to see instances where it has become unbalanced. For example, it was clearly the original intention of the Constitutional Convention and its Ratifying Legislatures that the national government should have only a limited role in adjudicating boundary disputes between the states, interstate commerce and the like, and fairly expansive powers to regulate international treaties and relations, but hardly anything more. Over a period of time, however, it is clear that the the central power has come to brush aside the powers of the states. The high point of the original power of States Rights was probably achieved in 1795 with the Eleventh Amendment. This Amendment, the achievement of Charles Ingersoll of Pennsylvania, forbids a citizen to sue a state in Federal Court. Since that leaves the citizen with only the right to sue a state in its own state courts, state power is clearly expressed in the right of the state to decide whether it wishes to be sued. Ever since that time however, state power has been steadily diminished, ultimately depriving the state legislatures of their power to select the United States Senators of their states in the Seventeenth amendment. With this steady loss of state power, it is possible that enough sovereignty has been lost to make it impossible to enforce the Eleventh amendment. This amendment was the product of the Progressive Movement of the late Nineteenth century, which now can possibly be seen to have reached its own reversal point in 2012, with Chief Justice John Roberts' opinion that the Federal Government may not dictate terms of state Medicaid programs through the threat of withholding Federal funds from the programs. It remains to be seen whether this change of direction is temporary, or the beginning of a trend. In any event, here it is demonstrated how the balance of power may be shifted, and long-standing imbalances may be corrected by small internal shifts, operating over long time periods.
Another major shift in power, almost the opposite of what the Founding Fathers passionately intended, is the increase in Presidential power. Although the Revolutionary War was fought about the excessive use of power by the British monarch, as much as about anything, there is now little question that the American President has more power over this nation than the British monarch has over his own. This trend can also be seen to have resulted from a series of small actions, over centuries of time. Many millions of Americans now listen intently to Presidential candidates presenting or refusing to present, "their" policies and programs for the following four years. The Articles of Confederation did not even provide for an executive branch, and the inclusion of one in the first draft of the Constitution was viewed with alarm by many citizens. It is hard to read the Constitution today without concluding that it was the original intent to leave policy to Congress, and implementation of that policy to the executive. Whether the Court will reverse this trend, whether the Congress will rebel against its loss of power, or whether we will continue toward an elected monarchy -- remains to be seen.
At the other extreme, we can seen the fiercely defended power of the various components of the Department of Defense to retain their independence. The Air Force had comparatively little difficulty establishing its independence of the Army Air Force, and it can be seen that some of this independence rests on the differing engineering requirements of airplanes, tanks and submarines, as taught in the service academies. But an argument can be made that the military services would be a stronger military force if they were combined in a single unit of warfare, and undoubtedly it is true that this would be a less expensive arrangement. Despite the unquestioned efficiencies, however, this separation of powers has so little chance of reversal that most congressmen would not consider it worth their time to investigate the merits.
In conclusion, it is not possible to be certain whether separation and balancing of powers is a good way to run a country or not. It is merely the way we have chosen to do things in this country. We are not likely to change it as long as we can boast we have the only durable Union of states the world has devised voluntarily. The strong inclination to continue a system which seems to work is likely to overpower any inclination to criticize it. In spite of the evidence that the system would permit further excursions toward the centralized government we are committed to resist. And in spite of evidence from the military forces that drift of this sort can be stopped if there exists the determination to stop it.