
A collection of essays on pivotal moments in American history includes Caleb Carr on America had there been no Revolution, and Robert Dallek on what might have happened if JFK had not been assassinated.
http://en.wikipedia.org/wiki/War_Plan_Red
Also, if you find that interesting you might like the exercise "Operation Skyshield" that had UK nuclear bombers successfully penetrating US air defenses in the 1960s:
http://en.wikipedia.org/wiki/Operation_Skyshield
Edit:
Also, the book "What Ifs? of American History" has a scenario with the US going to war with the British Empire in 1896 - with the apt title of "The Whale Against the Wolf".
http://en.wikipedia.org/wiki/What_Ifs%3F_of_American_History
In the aftermath, the U.S. military executes air strikes on Cuba followed by an invasion. The Soviet forces obliterate Guantanamo with a nuclear strike, send nuclear cruise missiles at the incoming invasion force,and, most seriously, manage to launch two of the SS-4 missiles...one of which hits Washington D.C. and wipes out the entire National Command Authority. In response, U.S. forces execute the entire SIOP against the Soviet Union, an effort which gives "overkill" a new name.
The aftermath of "The Two Days' War" includes the near-extermination of the Soviet Union, radiation issues in large parts of the world, and a "nuclear twilight" causing worldwide food shortages and famine. Ultimately, the United States was viewed as the aggressor by the rest of the world, compounded by the actions of President Richard Nixon (elected in 1964, replacing Acting President John McCormack). The U.S. stood alone in refusing to join the Geneva Convention for the Abolition of Nuclear Armaments in 1966, and renounced UN membership in 1968, ordering the organization out of New York City. The American public felt Nixon had taken the wrong turn, and elected Eugene McCarthy to succeed him in 1972.
The essay is written as the report of an investigative commission written in 1972 and finally declassified in 2002, in part by the actions of U.S. Archivist Newton Leroy Gingrich (who never went into politics in this timeline) at the New Capital District in Cheyenne Mountain, Colorado.
