The plan to attack France first was a plan by the German Chief of General Staff, Alfred Von Schlieffen which he came up with way back in 1905.
He decided that France was the enemy to be defeated first, with Russia held off until the French were annihilated. He worked out a detailed time timetable that took into account possible French responses to German actions. He believed that Germany would defeat France within six weeks, then after that he would turn his full force on Russia.
Schlieffen plan was heavily modified by his successor Moltke, thus during the outbreak of WWI in 1914, it was not implemented as Schlieffen had envisioned.
