In September 1939, Reichenau commanded the 10th Army during the German invasion of Poland and was the first German to cross the Vistula river, which he swam across. After the campaign he was awarded the Knight's Cross of the Iron Cross for his role as commander of the 10th Army. During the final two years before the outbreak of WWII, the majority of the German General Staff was so alarmed by the rush to war and so convinced of Germany's insuperable strategic disadvantages that they plotted to overthrow Hitler. When Reichenau learned of Hitler's plans to invade the Low Countries following so quickly on the heels of the Poland campaign, he opined this was "absolutely crazy wahnsinnig" and in November 1939 met with leaders of the military-tied German Resistance, not only revealing the invasion plans but going so far as to suggest means by which the Dutch might thwart the invaders. In spite of the efforts of Reichenau and others in the German Resistance and for various reasons, the Allies did not heed the warnings; Reichenau stepped back from his opposition to Hitler. In 1940 he led the 6th Army during the invasion of Belgium and France and later that year Hitler promoted him to field marshal during the 1940 Field Marshal Ceremony.