Expression used to denote the idea that 'once something has occurred, it is final'; 'once', 'as soon as', 'the moment'.
Long explanation:
The ~たが最後 grammar point is used to emphasize the finality of an action once it has been completed. In English, it can be translated as 'once', 'as soon as', or 'the moment'. It is often used to describe the start of an irreversible action or situation.