Dense fog came to the rescue of a military campaign on this day 250 years ago.

During the Seven Years’ War, the British were determined to oust the French from North America. The key to taking French colonies in what is now Canada was the capture of the great fortress at Louisbourg, on the coast of Nova Scotia. But things did not start well. A huge British invasion force of 12,000 troops in a fleet of 39 warships and 120 transport ships faced a rough Atlantic crossing that delayed the schedule.

When a landing was made near Louisbourg on June 8, extremely hazardous conditions in rough seas almost wrecked the invasion. In the days afterwards, a thick fog descended, as warm air on land collided with Arctic currents at sea.

Gradually the British tightened their grip around the fortress and shattered its defences. The last hope of the French rested on the guns of their remaining warships. But under cover of thick fog, on July 25, a raiding party crept into the harbour and captured the last two French ships and the French surrendered.

The fall of Louisbourg gave the British control of the entrance to the St Lawrence River and opened the way to the capture of Quebec the following year.