What kind of union do the states have?

The United States have a 'Federal' system of government, which means that, in theory, powers are divided between state and local, and Federal (central) authorities.

The US Government has evolved a great deal, structurally, since 1789-90, when the US Constitution setting up this arrangement first came into effect, but the momentum has generally been to centralize power more and more in the federal government, for several reasons.

First, and most cynically, power, once acquired, is often maintained by precedent, and it can be difficult for another body to challenge that power. But more specific to US History, the USC was a reaction to the 'Articles of Confederation,' which theoretically unified the USA during the 1780s after their independence, but proved to have too weak a central government, and no way of resolving disputes among states effectively. Thus, the USC was a compromise document set up to remedy this problem without creating opportunities for authoritarianism or a dictatorship. It gave specific and critical powers to the US Government, while still referring to the provinces (the typical Word in english for regions of a country) as 'states.' The Word 'State' suggests that a state has independence and political power.

The reality is more mixed. As US history progressed, managing a large and ever more complex (technologically and economically) society became more difficult, and more centralized government was needed. Thus, with several periods of rebellion against centralized power (the American Civil War was not one of them. Though many people believe this was a fight over states rights, the issue was not even raised until 1865 by Jefferson Davis after the war was lost. In any case, that argument is neatly dealt with by the following dialogue: 'the war was over states' rights? A state's right to DO WHAT?! It's also notable that the southern states pushed through the 'fugitive slave act,' which violated the laws of most northern states and demanded federal laws enforce slavery laws from the south beyond their territory). That digression over, a pro-business Supreme Court would not allow the Federal government to intervene in the economy between 1890 and 1932, roughly. They also invalidated the income tax as unconstitutional (a constitutional amendment reimposed it around 1913). However, from 1937, as Congress and the President struggled to deal with an economic emergency and the US Court began striking down important legislation (on very thin constitutional grounds, it mus be said) that the President believed imperiled the economy, the president strong-armed the court into allowing New Deal legislation through, paving the way for the New Deal to continue to provide millions of jobs for the unemployed, and, during the World War, take almost complete control over the economy. Since then, this organization was considered a settled question by legal experts, until, in the 2nd Bush administration, a 5 votes to 4 right-leaning supreme court started again to chip away at the Federal Government's authority. We have yet to see how this will play out. Most modern nations do not have 'Federal' central governments that can prevent them from taking what maybe necessary action, but many Americans believe limiting central government power is incredibly important, to limit tyrranyy