天てん下か分わけ目め

noun which may take the genitive case particle 'no'

decisive moment; fateful turning point; climactic battle

Used for a pivotal event, especially a battle or contest that determines the course of history or a major outcome. Often carries a dramatic, historical tone.

• 関ヶ原の戦いせきがはらのたたかいは天てん下か分わけ目めの戦たたかいだった。