The ratification of the U.S. Constitution in 1788 established the basis of the American political system as it operates today. The Constitution was drafted in response to the shortcomings of the Articles of Confederation, put into place by political leaders immediately after the Revolutionary War. Although amended through the years, the Constitution clearly outlines the foundations of American government.
Separation of Powers
The American political system prevents any one body of government from gaining too much influence through the separation of powers enumerated in the first few articles of the Constitution. Powers are divided between the executive branch, led by the President; the legislative branch, composed of the Senate and the House of Representatives; and the judiciary branch, or the Supreme Court. As delineated in the Constitution, duties of each of these branches balance the powers of the others and keep any single branch from dominating the government. This idea dates back to the ancient Greek and Roman republics.
While the U.S. is one country, it is also the union of 50 states. Early leaders of the new nation scrapped the original Articles of Confederation, which provided too little central authority; the almost complete independence of the different states weakened the national political system. The Constitution defines the responsibilities of the federal government, and then explicitly affirms that functions not delegated to or prohibited by the federal government are areas in which the states have full sovereignty within their own borders.
Individual liberties are an essential element of the American political system. An entire Bill of Rights -- the first 10 amendments to the Constitution -- eased public fear of a tyrannical government in the post-Revolutionary War period. The liberties enshrined in the Bill of Rights allow people to not only pursue their lives as they see fit, but also ensure they are free to participate in the democratic process. Specific rights outlined in the First Amendment -- freedom of speech, freedom of assembly and freedom to petition the government -- guarantee the political process does not bar citizens.
The American political system is a republic, not a true democracy. Every matter of government is not put to a direct vote of the citizenry, but handled by elected officials. Voting for an individual who you believe shares your values and views is how you directly affect the governance of the nation. The Constitution initially placed certain barriers between citizens and the direct selection of officials, such as the Electoral College; this body of representatives determined the president and vice president in early U.S. elections, but the procedure was changed in 1804 by constitutional amendment. The modern Electoral College still confirms electors' votes for president and vice president, but is a mostly ceremonial body.