The problem was that Russia was also friends with France, and the Germans thought the French might attack them to help Russia. So the Germans decided that they could win the war if they attacked France first, and quickly. They could mobilize very quickly. They had a list of all the men who had to join the army, and where those men had to go, and the times of every train that would carry those men to where they would have to fight. France was doing the same thing, but could not do it as quickly. The Germans thought that if they attacked France first, they could 'knock France' out of the war before Russia could attack them.