There's been a lot of fights between them so you're not being specific enough, since you said it's great britain it must be a war past May 1, 1707, I'm guessing it's the French Indian war in 1760, Great Britain received New France after the war.
There's been lots of conflicts between the two but I'd go with the most well known and say the French-Indian war where France lost New France, which include most of settled Canada at the time.