This pattern highlights a clear turning point, after which things become noticeably different. It's used when the change is significant and often irreversible, like a life event, a policy change, or a personal realization. The part before the boundary and after are usually contrasted, even if not explicitly stated.

It's a formal, written-style expression, common in news, essays, and literature. In conversation, you might hear it in serious discussions, but it sounds a bit stiff for casual chat. When used with verbs, you nominalize the clause with の or こと before attaching を境に, as in 結婚したのを境に.

Think of it as 'taking X as the dividing line'. The focus is on the fact that after that moment, the situation shifted permanently.