Battles & Wars › Between the wars
Second Sino-Japanese War
July 7, 1937 – September 2, 1945· War· Asia & Pacific
Well documentedSolid records, physical evidence or scientific consensus.
Japan's full-scale war on China from 1937, which merged into World War II and ended with Japan's surrender in 1945.
See it on the Battles & Wars map →What happened
A clash at the Marco Polo Bridge near Beijing on July 7, 1937, escalated into full war. Japan took Shanghai after three months of fighting, then the capital Nanjing, followed by massacre. Chiang Kai-shek's government retreated inland to Chongqing, trading space for time, while Communist guerrillas fought behind Japanese lines. Japan held the coast and cities but could not end the war. After Pearl Harbor it became part of World War II. Chinese deaths are estimated at roughly 15 to 20 million, mostly civilians.
Why it matters
It tied down much of Japan's army, drove Japan toward Southeast Asia's oil and war with the US, and left the Communists stronger for the civil war that followed.
How we know
Japanese and Chinese military records, the Tokyo war-crimes trial, foreign journalists' reports, and population studies of wartime losses.
Still debated
Total Chinese deaths are uncertain, and the relative roles of Nationalists and Communists in resisting Japan are still argued, partly for political reasons.
Inside this (3)
Connections (5)
▸Mukden Incident and invasion of Manchuria led to Second Sino-Japanese WarWell documented
The connection
Seizing Manchuria gave Japan a base, resources and a model of army-led expansion. In the early 1930s it pushed into Rehe and northern China, creating buffer zones near Beijing. Chinese anger grew, and Chiang Kai-shek finally agreed to unite with the Communists against Japan. When troops clashed at the Marco Polo Bridge in 1937, neither side backed down.
When and how
Northeast and north China, 1931–1937, from Mukden to the Marco Polo Bridge near Beijing on July 7, 1937.
How we know
The chain of incidents and truces between 1931 and 1937 is well documented; China officially counts a 'Fourteen-Year War' from 1931.
Debates and open questions
Whether 1937 was the inevitable outcome of 1931, or an escalation neither government planned, is debated.
▸Nanjing Massacre was an atrocity committed early in Second Sino-Japanese WarWell documented
The connection
After heavy losses taking Shanghai, Japanese armies raced to Nanjing, the Nationalist capital, expecting its fall to force China's surrender. Instead the massacre hardened Chinese resistance and drew international condemnation. Chiang moved the government inland, and the war went on for nearly eight more years.
When and how
Nanjing, December 1937 to January 1938, by troops of Japan's Central China Area Army under General Matsui Iwane.
How we know
Western residents' diaries, Japanese soldiers' records and the postwar Tokyo and Nanjing tribunals; Matsui was convicted and executed in 1948.
Debates and open questions
Why discipline collapsed so completely, and whether killings were ordered or merely tolerated, is still studied; the death toll remains disputed.
▸Second Sino-Japanese War merged into World War IIWell documented
The connection
Japan's war in China, begun in July 1937, tied down most of its army and drew Western sanctions. When Japan attacked the US and Britain in December 1941, China formally declared war on Japan, and the Asian and European wars fused into one global conflict. China fought on until Japan's surrender in 1945.
When and how
China, from the Marco Polo Bridge incident near Beijing on July 7, 1937; merged into the wider war after Pearl Harbor, December 1941.
How we know
Chinese, Japanese and Allied records; China signed the January 1942 Declaration by United Nations as one of its four leading powers.
Debates and open questions
Many Chinese historians date World War II's start to 1937, or even to Japan's 1931 seizure of Manchuria, rather than to Europe's 1939; the start date depends on perspective.
▸Second Sino-Japanese War led, via the US oil embargo, to Attack on Pearl HarborMostly agreed
The connection
To cut China's supply routes and gain resources, Japan moved into French Indochina in 1940–41. In July 1941 the US froze Japanese assets, which halted the oil sales Japan depended on. Facing shrinking reserves and US demands to leave China, Japan's leaders chose war, striking Pearl Harbor to protect a drive on the oil of the Dutch East Indies.
When and how
Washington and Tokyo, July–November 1941: the asset freeze followed Japan's move into southern Indochina; the November 26 Hull note demanded withdrawal from China.
How sure are we?
Japanese imperial conference records from September to December 1941 and US diplomatic papers show the oil embargo and China question driving the decision.
Debates and open questions
Whether US pressure left Japan no real choice, or Japan could have compromised on China, is debated; the total oil cutoff may have gone beyond Roosevelt's intent.
▸Second Sino-Japanese War paused and then reshaped Chinese Civil WarWell documented
The connection
Nationalists and Communists formed a second United Front in 1937, but it broke down in clashes by 1941. Japan's invasion bled the Nationalist armies and economy, while the Communists expanded behind Japanese lines, from tens of thousands of troops to about 900,000 regulars by 1945. When Japan surrendered, both sides raced to take its territory.
When and how
Across China, 1937–1945; the New Fourth Army Incident of January 1941 ended real cooperation, and full civil war resumed in 1946.
How we know
Records on both sides and reports by US observers, such as the 1944 Dixie Mission to the Communist base at Yan'an, document the shifting balance.
Debates and open questions
Whether the Communists would have won without the Japanese invasion is a central 'what if' of modern Chinese history.