εἰ μὴ διά

From LSJ

ἔξαψις σφοδρὰ μετὰ πολλῆς βίας πίπτουσα ἐπὶ γῆς → a violent flare-up falling on the ground with great force, thunder and lightning

Source

Greek > English (Woodhouse)

but for, had it not been for