critical, decisive, turning-point (attr.), watershed (attr.)
watershed
Этот хребет образует водораздел двух больших рек.
This mountain ridge forms the watershed between two major rivers.
Мы стояли на водоразделе и смотрели вниз.
We stood on the watershed and looked down.
Война стала водоразделом в истории страны.
The war became a watershed in the country’s history.