Why was quickly defeating France a key element of Germany's military strategy before World War 1?

Because Germany needed to defeat France on the Western front before Russia could prepare to strike on the eastern front.

Explanation:

The battle for France was the episode that resulted in the conquest of the Nazis over France. The Nazi victory happened in about 46 days, and the speed of the conquest surprised the world, as it was expected that the French resistance would be much greater. The invasion of France was one of the great feats of the Nazi army and paved the way for new conquests to be made throughout the war.

The conquest of France was strategic for Nazi Germany, mainly because France posed the greatest threat to Germany in Continental Europe and defeat it would allow Hitler to isolate the United Kingdom in the war. Moreover, the defeat of France and, later, of the United Kingdom would allow Hitler to focus on his great goal: the conquest of the Soviet Union and the destruction of Bolshevism.

To avoid fighting on two sides with Russia to the East and France to the West