BBC Radio 4's In Our Time program for May 8, 2014, discussed the Second Sino-Japanese War.
The Japanese army in China used the case of a missing Japanese soldier to increase armed aggression in China in what became known as the Marco Polo Bridge Incident on July 7–9, 1937, when Chinese republican forces and Japanese imperial forces crashed at a bridge written about by Marco Polo in the 1200s.
In fact, the soldier, Private Shimura Kikujiro, had not been killed or kidnapped as the Japanese commander thought, but had wandered off to relieve himself.
Though it is an oversimplification to call this curious case of a missing soldier a direct cause of 23 million deaths, it was the final significant incident triggering the Second Sino-Japanese War (1937–1945), which claimed approximately 24 million human lives, 22 million of those being Chinese civilians, approximately 480,000 being Japanese military personnel.
From the In Our Time episode's Web page:
Melvyn Bragg and his guests discuss the Sino-Japanese War of 1937-45. After several years of rising tension, and the Japanese occupation of Manchuria, full-scale war between Japan and China broke out in the summer of 1937. The Japanese captured many major Chinese ports and cities, but met with fierce resistance, despite internal political divisions on the Chinese side. When the Americans entered the war following the bombing of Pearl Harbor, the Japanese found themselves fighting on several fronts simultaneously, and finally capitulated in August 1945. This notoriously brutal conflict left millions dead and had far-reaching consequences for international relations in Asia. With: Rana Mitter, Professor of the History and Politics of Modern China at the University of Oxford; Barak Kushner, Senior Lecturer in Japanese History at the University of Cambridge; and Tehyun Ma, Lecturer in Chinese History at the University of Exeter
The architecturally remarkable 11-span bridge is located near the village of Wanping about 9.3 miles southwest of Beijing's city center. In the Pinyan ("spelled-out sounds") system of Chinese using Latin characters it is the Lúgōu Qiáo bridge. It was built during the years 1189–1192 and extensively repaired and in large parts reconstructed in 1698. A few of its approximately 500 decorative lions date back to the original 12th-century structure.
Japan's victory over Russia in the Russo-Japanese War (1904–1905) shocked the Western world. In the conflict's wake, the Japanese government claimed control of railway-related rights that China granted to Russia in the 1896 Li-Lobanov Treaty. As the years passed, the presence of the Japanese army in China grew, as did the meddling of the Japanese government and army in various Chinese affairs. In 1931, rogue Japanese military personnel staged the "Manchurian Incident" or "Mukden Incident" as a pretext for the Invasion of Manchuria that year. (The invasion is part of the dramatic backdrop for the film The Last Emperor).
The Second Sino-Japanese War was primarily a conflict between rival Chinese nationalist and Communist forces in armed resistance against the Japanese army on Chinese soil, though it merged with the Second World War's Pacific Theater, with Germany, the Soviet Union, and the United States of America all becoming involved in the Sino-Japanese conflict proper to lesser degrees. Parts of the conflict spilled over into French Indochina, too.
The war was horrific. The Japanese forces used chemical and bacteriological and chemical weapons--including Japanese Army Air Force bombs of fleas carrying the bubonic plagues. The war's most notorious atrocity was the "Rape of Nanking" during which anywhere between about 40,000 to more than 200,000 Chinese non-combatants were killed over a six-week period beginning December 13, 1937.