Following the third world war it was propperly wrecked with the atom bombs, as was everywhere else, so to survive, and to stop any other war like it, every leader most likely decided to abandon there inderpendace and form one united land.
Yes, this is the most likely reason why. It has been theorized that the Vulcans assisted with advanced methods of radioactive fallout cleanup to lessen radiation hazards and nuclear winter, which would have been devestating even in the "limited" scenario the real-world experts have said would never happen. The idea is that Humanity could collectively survive a "regional" nuclear conflict, with only a few hundred million, out of several billion, dead.
One would think that as a major nuclear arms power, the U.S. would likely be a prime target, Cold War or no; however, it does seem that in Star Trek, it was a "limited" or "regional" conflict that the U.S. theoretically could have stayed out of. However, I suppose it is possible, say, for a nuclear war between India and Pakistan to result in, say, sixty percent casualties, limited to the Indian subcontinent. But then it seemed to me that radioactive fallout and nuclear winter would be an issue, as weather systems spread it worldwide, even for the majority unaffected by the immediate destructive power of impacting and airbursting warheads.
It's also possible that American exceptionalism, which even now is on the wane, had diminshed to the the point where Americans were simply amenable to joining a real pan-World government structure ProfessorTrek 14:12, August 15, 2011 (UTC) =
wouldn't USA joining United Earth go against the constitution?
Well first you're applying a real world concern to a fictional scenario. More importantly I can't imagine anyone trying to use the constitution to stand in the way of world peace. Lastly in reality it would not go against the constitution to begin with.
Politically, that may or may not be, post-WWIII, true. That it would not "go against" the Constitution is inappropriately conclusory.
Art. 4, Sec. 4, Cl. 1 sets forth that "[t]he United States shall guarantee to every State in this Union a Republican Form of Government..." this would tend to suggest that the U.S. States could not be subsumed in a larger government (remember the dual sovereign character of American federalism). In Federalist No. 10, it was asserted that the Founding Fathers wanted to avoid pure democracy in favor of something somewhat reminiscent of Platonian republicanism.
However, in Luther v. Borden, 48 U.S. 1 (1849) the Court held that "it rests with Congress to decide what government is the established one in a State ... as well as its republican character,” and thus held the clause non-justiciable, and vested the power with the U.S. Congress.
Theoretically, it could be up to Congress to decide. However, balancing that are Amendment IX and Amendment X, which vest rights not specifically enumerated in the Constitution to the People, and which allows vestiture of rights not specifically enumerated nor specifically prohibited to the States reserved to the States or to the People, respectively. My guess is the Court might rule that since the Guarantee Clause is non-justiciable, but residual rights are vested in the People or the States, that this decision might warrant what might be the first-ever national plebiscite or binding referendum, and order that direction on this issue, more important even than a Constitutional amendment (which arises from representative democracy rather than pure democracy) can only arise from direct and specific advice to the Congress from the People. This still might not be considered a function of pure democracy, but instead one of direct democracy, because all this does is give a binding mandate to Congress and might not constitute the People themselves specifically enacting a law. Keep in mind that politically, for any other type of issue, this probably would be considered, politically if not legally, un-American.
Of course, if the precursor to United Earth was the United Nations, then the U.S. would remain at least a focal point for any kind of world government that would be an outgrowth from it. Then again, the "new United Nations" (ca. 2079) might not have been based in New York City (ST:TNG "Encounter at Farpoint") ProfessorTrek 18:57, August 21, 2011 (UTC)