[gh-AIR] (n. f.) The 10th century Normans, those conquesting descendants of Vikings conquerors (the apple doesn't fall far from the tree...), were the first to take early French and start making it sound like what English is today. While the rest of France was speaking Françoys, the Norman were altering the pronunciation of words and taking those with them to vanquished England.
It's fitting to consider the French word guerre. While most of the country was pronouncing that "gh-AIR" with a hard G, the Normans in their province of Normandy saw that "gue-" and thought it sounded more like "gweh" to them. That Norman accent was also responsible for transforming the French surname "Guillaume" into "William." The rest was just a process of simplification: from guerre to "gwerre" to "werre" and finally war.