Again, Alternate History isn't exactly a new concept; Roman historian Livy, during the 1st century CE, wrote a treatise examining what might have happened if Alexander the Great had chosen to invade Europe rather than the Middle East.
Winston Churchill also wrote an interesting short tale in the form of an alt-hist scholarly document, "What If Lee Had Lost the Battle of Gettysburg?", written from the perspective of a historian in a world where the Confederacy had won the Civil War.
Generally, though, the ur-example is thought to be Philip K. ****'s The Man In the High Castle, published in 1963, and taking place in a world where FDR was assassinated during his first term, and President Garner refused to assist either Britain or the USSR. After the Nazis took Europe, they turned their attention to the US, which they conquered in 1948. Interestingly, this novel includes its own alt-hist novel mentioned in the text, [/]The Grasshopper Lies Heavy[/i], in which the Axis lost, and the world was in the grip of a prolonged Cold War between the US and Great Britain.
I quote myself because I just realized the major example I forgot before: Larry Niven's "All the Myriad Ways", published in 1968.
Again, it's not exactly original to Trek.
------------------------------------------- I'm old enough not to care too much about what you think of me --
But I'm young enough to remember the future, the way things ought to be... - Rush, "Cut To the Chase", Counterparts