Looking over the Star Trek timeline only up to when TOS (the original series) ended in 1969 it is quite obvious that Star Trek is an AH. I used the end of the show as the cut off as possible futures don't generally get the AH label.
If we exclude the immortal Flint, the ancient astronauts, and very minor events we get these divergences:
* Rome had no sun worshipers ("Bread and Circuses")
In OTL Rome, in fact, had several cults that revered sun gods including Helios Apollo and Sol from the Greeks, Mithras from the Persians, and Elagabalus from the Syrians. Then from the 3rd century on there was Sol Invictus ("Unconquered Sun") which may have been a new sect or a revival of an old one. So two of these gods in English would have been called "Sun".
* WWI
In Star Trek's history only 6 million died in WWI. ("Bread and Circuses") In OTL over 41 million died in WWI.
* WWII
In Star Trek's history reality only 11 million died in WWII. ("Bread and Circuses") In OTL the range is 50 to more then 80 million.
* The Nazis were close to inventing the atomic bomb and would use the V2 rocket to deliver it. ("City on the Edge of Forever")
I explained this in detail in the Edith Keeler Lives: A brief history thread
* Nazi Germany was the most efficient state Earth every knew ("Patterns of Force")
Disproved in Rise and Fall of the Third Reich (1960)
* 1967
The Outer Space Treaty of 1967 is not signed allowing the United States to launch an orbital nuclear warhead platform to counter a similar launch by other powers in 1968.("Assignment: Earth") In OTL only the USSR launched such platforms as part of the “Fractional Orbital Bombardment System” or FOBS.
* 1968
The United States efforts to counter FOBS is interfered with by Gary Seven to scare it and the USSR out of the arms race. Thanks to interaction with the Enterprise the US version of FOBS explodes exactly one hundred and four miles above the Earth.("Assignment: Earth") In OTL the treaty of 1967 prevented this project from even existing.
If we exclude the immortal Flint, the ancient astronauts, and very minor events we get these divergences:
* Rome had no sun worshipers ("Bread and Circuses")
In OTL Rome, in fact, had several cults that revered sun gods including Helios Apollo and Sol from the Greeks, Mithras from the Persians, and Elagabalus from the Syrians. Then from the 3rd century on there was Sol Invictus ("Unconquered Sun") which may have been a new sect or a revival of an old one. So two of these gods in English would have been called "Sun".
* WWI
In Star Trek's history only 6 million died in WWI. ("Bread and Circuses") In OTL over 41 million died in WWI.
* WWII
In Star Trek's history reality only 11 million died in WWII. ("Bread and Circuses") In OTL the range is 50 to more then 80 million.
* The Nazis were close to inventing the atomic bomb and would use the V2 rocket to deliver it. ("City on the Edge of Forever")
I explained this in detail in the Edith Keeler Lives: A brief history thread
* Nazi Germany was the most efficient state Earth every knew ("Patterns of Force")
Disproved in Rise and Fall of the Third Reich (1960)
* 1967
The Outer Space Treaty of 1967 is not signed allowing the United States to launch an orbital nuclear warhead platform to counter a similar launch by other powers in 1968.("Assignment: Earth") In OTL only the USSR launched such platforms as part of the “Fractional Orbital Bombardment System” or FOBS.
* 1968
The United States efforts to counter FOBS is interfered with by Gary Seven to scare it and the USSR out of the arms race. Thanks to interaction with the Enterprise the US version of FOBS explodes exactly one hundred and four miles above the Earth.("Assignment: Earth") In OTL the treaty of 1967 prevented this project from even existing.