Historians label an event as a turning point when it

A turning point is any moment in history in which a significant change occurs.

The American Revolution is a turning point in World History and US history because of the impact of a colony overthrowing its colonial overlord.

In History, a turning point refers to a particular and significant event that unleashes a major historical development that may change the course of events.  

An example of a turning point in history is the division of Islam between Sunni and Shia Muslims, which caused a series of conflicts that remain nowadays.

i confess that there are several parts of this constitution which i do not at present approve, but i am not sure i shall never approve them: for having lived long, i have experienced many instances of being obliged by better information or fuller consideration, to change opinions even on important subjects, which i once thought right, but found to be otherwise. it is therefore that the older i grow, the more apt i am to doubt my own judgment, and to pay more respect to the judgment of others.