The defense of the Great Wall (simplified Chinese: 长城抗战; traditional Chinese: 長城抗戰; pinyin: Chángchéng Kàngzhàn) (January 1 – May 31, 1933) was a campaign between the armies of Republic of China and Empire of Japan, which took place before the Second Sino-Japanese War officially commenced in 1937. It is known in Japanese as Operation Nekka (熱河作戰, Nekka Sakusen) and in many English sources as the First Battle of Hopei.