How would it most likely come about? In response to crisis, most likely. Imagine not one, but several successive World Trade Center attacks, over a period of years, and with nuclear weapons. Now imagine that repeated instances of inaction, infighting or incompetence on the part of civil authorities being proven to have contributed to those attacks. Then imagine a military figure -- charismatic, ambitious, competent -- coming to public attention by dealing successfully with a public crisis.
Now imagine a weak and vacillating president, appointing such a person to a vacancy in the vice-presidency to shore up his popularity. In the next crisis, a group of young military officers, comrades of our new Napoleon, start to wonder whether they can tolerate weakness in the White House leading to another catastrophe in an American city. Perhaps a not-entirely-explainable death; perhaps a sudden resignation. These things happen at such times in empires.
The federal system would have to go, of course. Empires do not tolerate multiple, independent power centers. Multiple attacks of a severe nature would probably create an ongoing depression and emergency economy. It would be easy for our newly elevated commander in chief to burden the states with enough unfunded mandates to drive them into bankruptcy. In response to this crisis, a plan for a fusion of federal and state authorities, and debts, might be something state legislators would end up reluctantly approving. A new constitution would be adopted by national plebiscite, with the integrity of the vote being guaranteed by the military.
Of course the new constitution would have local assemblies of some sort, probably based on existing federal administrative regions. Both the regional assemblies and new national Congress would probably be elected by pure proportional representation, so as to assure fragmentation into many small ethnic-based parties, easy to manipulate and assemble into coalition governments.
Many people would be surprised by the liberal and progressive nature of the empire's domestic policy. Multiculturalism would be retained and enhanced, and the country would probably be declared officially bilingual in English and Spanish, the better to annex Latin American states. Again, the more divided the citizenry, the easier it is for a strong executive to manipulate them. Surprisingly to some, the neo-Confederate movement in the South would be quietly encouraged as a cultural movement, within limits, again to divide sentiments.
Tax policies would be fairly leveling, and a strong safety net would be maintained, all the better to create a healthy lower middle class as a source of soldiers. Existing business would be protected; new ones would be much harder to start.
Education would be nationally uniform, highly meritocratic, with a strong military content to schools. Elite schools would resemble military academies; schools for less-promising students would range from boot camp to reform school. Prisons would be emptied into military penal battalions for the more tractable, and work camps in frigid backlands for the less so. Domestic security police would actively intervene into domestic politics in a way that would make tomorrow's radicals view the old FBI with nostalgia.
Foreign policy would be radically different from the present. "Blood for oil" would not be an anti-war slogan; it would be an acknowledged government strategy. Probably, the government would conclude that if blood were to be spilled for oil, it should be done in the most cost-effective manner: Canada, Venezuela, and Mexico would be annexed outright and integrated as new regions into what might be renamed the United State of the Americas; Colombia and Ecuador would be annexed to guard their security. The Middle East would be ignored and Israel abandoned.
Caribbean and Central American tax havens would merely be informed of their annexation dates by their arriving governors. Guerilla campaigns against these acquisitions would be dealt with by a combination of high-tech weaponry, bribery, intimidation of neighboring states to deny cross-border sanctuaries, and mass transfer of populations sympathetic to rebels. Venezuelan patriots and Colombian drug lords alike would freeze building roads by the Arctic Ocean.
The empire would probably cut a deal either with Russia, to defend western Siberia in return for the eastern half, or with China, to acquiesce in their annexation of half of the same in return for America getting its cut. Negotiations with both parties would probably go on to the last hour. In Asia, the Philippines, and perhaps some other offshore areas, would be offered union with the empire, and would be leaned upon to accept.
Defense research would be strongly supported, and NASA would be incorporated into the Air Force. Space would be heavily militarized and occupied, with foreign satellites having to obtain launch clearance from the empire's authorities. Al Jazeera's content would change substantially.
Such an empire would be militarily strong, economically close to self-sufficient, and would be able to solve some internal social problems that now seem intractable. Such an empire would gravitate to being a semi-authoritarian state, although probably not a totalitarian one. More than a few people today who claim to fear empire would find they could tolerate it quite well. Even some libertarians would probably be happy with the five seats in Congress proportional representation would give them.
However, such a state would not be America. It would also likely lose the dynamism America now enjoys, in many ways more than ever before. Ultimately, such a structure would gradually eat away at the underpinnings of strong civil society such as America now enjoys, and it would become very difficult to regain it once it were lost.
To look at such empire both tells us how far America still is from yet being one, and what the stakes are in preventing the kind of stresses on America's existing civil society than would bring on such an emergency state. The alternative to strong action by a constitutional, democratic state against nuclear-armed terrorism is not life as before; it is something most people who grew up with today's America wouldn't care for.