The Marco Polo Bridge Incident in July 1937 led the Japanese to invade China, and Shanxi was one of the first areas the Japanese attacked. When it became clear to Yan that his forces might not be successful in repelling the Japanese army, he invited Communist military forces to re-enter Shanxi. Zhu De became the commander of the Eighth Route Army active in Shanxi and was named the vice-commander of the Second War Zone, under Yan himself. Yan initially responded warmly to the re-entry of the arrival of Communist forces, and they were greeted with enthusiasm by Yan's officials and officers. Communist forces arrived in Shanxi just in time to help defeat a decisively more powerful Japanese force attempting to move through the strategic mountain pass of Pingxingguan. The Battle of Pingxingguan was the largest battle won by the Communists against the Japanese.