The federal government, under the powers of the U.S. Constitution, is given the power to make laws, veto laws, oversee foreign policy and national defense, impose tariffs, impeach officials, enter into treaties, interpret the Constitution, interpret laws and revise laws that allow one state to impede on the rights of another. Beyond that, the 10th amendment gives power to the states to govern themselves.
Although the U.S. Constitution outlines general guidelines for issues that fall under state versus federal rule, there are areas of ambiguity that have sparked controversial power struggles. It was intentionally written to avoid providing one arm of government with too much power over the other. Since its inception, however, there have been many circumstances where it's unclear which government should have the final say. One of the functions of the Supreme Court is to interpret state versus federal power, but even the laws that grant the Supreme Court its power lack the clarity to avoid accusations of abuse of power. The United States, by design, is a nation of "united" states, meaning that the power of the federal government was intended to be limited. One of the most hotly contested issues in politics, however, is big government versus state government and which entity should have more power.