を境(さかい)にAfter (that moment), From (that time)

を境(さかい)に is a somewhat formal phrase used when a notable change has occurred. It is made up of the case-marking particle を, the noun 境(さかい) (meaning 'boundary' or 'border'), and the case-marking particle に. It may also be seen as を境(さかい)にして, with the て-form of する.

を境(さかい)に can be taken to more literally mean 'with (A) as a border', and is most often used with significant or radical changes, emphasizing a clear before and after, relative to a single point in time. As would be expected due to the use of the case-marking particle を, を境(さかい)に will directly follow a noun.

を境(さかい)に is commonly seen with points in time, however it may also be seen with specific events or things which have a clear 'border', such as specific sums of money, in some contexts.