How Mary defended the city of Lwow, Ukraine

A strange atmospheric phenomenon in Lwow, Ukraine, attested by numerous witnesses, gave a military victory to King John Sobieski, who had come to deliver the city besieged in 1675 by a Turkish army of overwhelming superiority. With an unequal match of 300 000 Turkish soldiers against 6000 hussars, the outcome seemed hopeless.

Before the battle, the whole population of Lwow knelt in the Cathedral before the image of the Merciful Virgin. As soon as the battle started, the left wing of the Polish army was under such severe attack that it would have been crushed if the sky had not suddenly darkened with thick clouds and a violent storm come down on the enemy with such power of thunder and lightning, rain and wind, that the Turks turned back in panic, while the Hussars, propelled on by the storm, chased after them shouting: "Long live Jesus! Long live Mary!"

This happened on the vigil of the feast of Our Lady of Czestochowa, celebrated on August 25.

Encyclopedia Maria volume IV 

Beauchesne 1956 - p. 695