The American presidency is designed to disappoint. Each candidate must promise things that are beyond his power to deliver. No candidate could expect to be elected by emphasizing how little power the office actually has and how voters should therefore expect little from him. So candidates promise great, transformative programs. What the winner actually can deliver depends upon what other institutions, nations and reality will allow him. Though the gap between promises and realities destroys immodest candidates, from the founding fathers' point of view, it protects the republic. They distrusted government in general and the office of the president in particular.
Congress, the Supreme Court and the Federal Reserve Board all circumscribe the president's power over domestic life. This and the authority of the states greatly limit the president's power, just as the country's founders intended. To achieve anything substantial, the president must create a coalition of political interests to shape decision-making in other branches of the government. Yet at the same time -- and this is the main paradox of American political culture -- the presidency is seen as a decisive institution and the person holding that office is seen as being of overriding importance.
Constraints in the Foreign Policy Arena
The president has somewhat more authority in foreign policy, but only marginally so. He is trapped by public opinion, congressional intrusion, and above all, by the realities of geopolitics. Thus, while during his 2000 presidential campaign George W. Bush argued vehemently against nation-building, once in office, he did just that (with precisely the consequences he had warned of on the campaign trail). And regardless of how he modeled his foreign policy during his first campaign, the 9/11 attacks defined his presidency.