1.
decisive battle; make-or-break moment
A contest whose outcome settles everything, leaving no second chance for the loser. Literally 'the dividing point of the realm', from the historical struggles that decided who would rule Japan. Used with の before the noun it modifies.
Nearly always used attributively with の, above all in the set phrase 天下分け目の戦い. The historical reference point is the Battle of Sekigahara (1600), which decided who would rule Japan; modern uses transfer that weight to elections, sports finals, and business deals, often with a touch of deliberate grandiosity.