These ideas are all critical to the political organization of the United States, as envisioned by the US Constitution. One thing to remember is that the Constitution was not the first political framework created for the United States. The first was the Articles of Confederation (which placed greater power in...
These ideas are all critical to the political organization of the United States, as envisioned by the US Constitution. One thing to remember is that the Constitution was not the first political framework created for the United States. The first was the Articles of Confederation (which placed greater power in the individual states than in the federal government). The task of the Constitution, then, was to envision a new system of government that would place greater power with the federal government (to more effectively meet the challenges of independence). At the same time, however, limits needed to be ensured, because if the federal government became too powerful, it could potentially become a tyrannical force. These three ideas are all intended to navigate this careful balance.
In federalism, power is shared between the state and federal governments. It is a political middle ground between unitary systems of government, where the central government is supreme, and a confederation, which would place political supremacy with the states. I would say that one of the strongest expressions of this principle in the US Constitution is the Tenth Amendment, which reserves all powers not given to the United States within the Constitution specifically to the individual states or the people themselves.
Meanwhile, the ideas of separation of powers and of checks and balances are linked with one another, to the point that they can be easily discussed together. Ultimately, government involves, by its very nature, the concentration of power and authority. The idea of separation of powers is to divide that power and authority into the different branches of government: executive, legislative, and judicial. With checks and balances, the idea is that within this division of power, the different branches can serve to balance one another to keep any one branch from becoming too powerful and a threat to liberty.
Thus, for example, consider the legislative process in the United States: only Congress has the power to pass legislature, but the president does have veto power within that process (though it can be overruled if there is a large enough majority). Similarly, while the president can negotiate treaties and make political appointments, the president at the same time requires Congressional approval to do so.