Short answers, pulled from the story.
The Defense of the Great Wall was a military campaign fought from the 1st of January to the 31st of May, 1933, between the armies of the Republic of China and the Empire of Japan. It took place after Japan's invasion of Manchuria in 1931 and before the Second Sino-Japanese War officially began in 1937. Japan seized the province of Rehe and extended its Manchukuo puppet state to the Great Wall.
The Japanese garrison commander at Shanhaiguan staged an incident on the night of the 1st of January 1933 by detonating hand grenades and firing shots, giving the Kwantung Army a pretext to demand the Chinese 626th Regiment evacuate the pass. When the Chinese refused, the Japanese 8th Division attacked with four armored trains, ten tanks, bombers, and shelling from a dozen warships of the IJN 2nd Fleet. Chinese commander Shi Shian was forced to withdraw on the 3rd of January after losing half his force.
Emperor Hirohito approved the operation against Rehe after the army's Chief of Staff requested his sanction, hoping it would bring the Manchurian matter to a close. He gave his approval on condition that the army was not to go beyond the Great Wall of China. The offensive launched on the 23rd of February, 1933, and Rehe's capital, Chengde, fell on the 4th of March.
Many Chinese units defending the Great Wall were equipped primarily with handguns, hand grenades, and traditional Chinese swords. They had limited supplies of trench mortars, heavy machine guns, light machine guns, and rifles, putting them at a significant disadvantage against Japanese forces armed with tanks, armored trains, aircraft, and naval gunfire support.
The Tanggu Truce, signed on the 22nd of May, 1933, created a demilitarized zone extending one hundred kilometers south of the Great Wall that Chinese forces were prohibited from entering. Japan was permitted to use reconnaissance aircraft and ground units to monitor Chinese compliance. China was also required to acknowledge the de facto independence of Manchukuo and the loss of Rehe.
The First Battle of Hopei is the name used in many English sources for the Defense of the Great Wall, the 1933 campaign between China and Japan. It is also known in Japanese by a corresponding term. The campaign ended with Japan absorbing Rehe into Manchukuo and imposing a large demilitarized zone south of the Great Wall on China.