I'd read the two previous volumes in this series, which are more global and less American in scope; loved the first one, less impressed by the second. This one concentrates on US history, and is generally pretty good - the one real dud is an essay on "What if Pearl Harbour hadn't happened?" which concludes that nothing would have been very different except that the Pacific War would have been six months late. The other Second World War essay is a bit more exciting but also concludes that it wouldn't have made much difference if Eisenhower had gone for Berlin.
There are no less than four essays on the Civil War, one of which is James McPherson's reprint from the first volume on "What if the South had won?", but the other three taking interesting tacks: one by the dubious Victor Davis Hanson credits Lew Wallace's personal disgrace at the battle of Shiloh with his later creation of the popular epic novel in Ben-Hur; one looking at the potential for insurrection against the Lincoln administration in what we now call the Mid-West, and one speculating (a bit chaotically) about the possibilities for continued insurgency in the context of Andrew Johnson as well as Abraham Lincoln being assassinated.
Two of the pieces are written from the counterfactual perspective first used, I think, by Winston Churchill in his 1931 essay "If Lee had not Won the Battle of Gettysburg". The one on how the Cuban missile crisis turned into a global nuclear war is rather conventional stuff; but Andrew Roberts' piece explaining the origins and course of the 1896 war between the USA and Britain is the pick of the book for me, although I don't quite agree on the likelihood of the US being given Quebec in a peace settlement; much more likely what happened in the 1885 Serbo-Bulgarian War, both sides being returned to the status quo ante.
The other piece that particularly caught my eye was on John Tyler, the first Vice-President to succeed to the Presidency after the death of his running-mate. Tom Wicker points out that Tyler's accession was far from assured by a strict reading of the constitution, and that the policies he pursued in office, in particular on the annexation of Texas, were crucial in their importance to the future of the country and not likely to have been pursued as successfully by any other potential president of the day. Tyler is much more interesting than I had realised, and the story has an exploding cannon as well, which in February 1844 killed numerous senior officials, one of whose grieving daughters found comfort in the arms of the recently widowed President Tyler, who married her four months later. (One of their grandsons is still alive.)
Anyway, a good collection for the history buff.