Matsui retired from active duty in 1935 but was called back into service in August 1937 at the start of the Second Sino-Japanese War to lead the Japanese forces engaged in the Battle of Shanghai.

Though he was a short, thin, and sickly young man, Matsui opted for a career in the Army, because in Japan at that time military schools charged the lowest tuition fees.

Matsui was a fervent admirer of the recently deceased Sei Arao (1858–1896), a "continental adventurer' (tairiku rōnin) and pan-Asianist army officer from his hometown who had served in China.

[9][10] Arao believed that China and Japan, as the two strongest powers in Asia, had to forge a close trading and commercial partnership under Japanese hegemony to resist Western imperialism, an idea which Matsui incorporated into his own worldview.

In 1921 Matsui was posted to Siberia as a staff officer, but returned in 1922 to China where he served until 1924 as an advisor to Zhang Zuolin in the Chinese city of Harbin and did intelligence work for Japan's Kwantung Army.

[17] As Chief of the Intelligence Division, Matsui was a strong supporter of Chiang Kai-shek, who was attempting to end the civil war in China and unify the country under his leadership.

[9] At the time Matsui was back in Japan commanding the 11th Division, but at the end of the year he was sent to Geneva, Switzerland, to attend the World Disarmament Conference as an army plenipotentiary.

[4] At first Matsui condemned the invasion as the work of renegade army officers, but he was equally stung by what he believed were unfair denunciations of Japan itself by Chinese delegates to the League of Nations.

[9][21] After returning to Japan in late 1932, Matsui abruptly appeared at the office of the Pan-Asia Study Group, a Tokyo-based think tank, and presented its members with a bold plan to expand their small organization into an international mass movement.

[22] Matsui persuaded them to adopt his ideas, and in March 1933 the study group was rechristened the Greater Asia Association (大亜細亜協会 Dai-Ajia Kyōkai), described by the historian Torsten Weber as "the single most influential organization to propagate pan-Asianism between 1933 and 1945.

[29] Between October and December 1935 he toured the major cities of China and Manchukuo speaking to Chinese politicians and businessmen about pan-Asianism and setting up a new branch of the Greater Asia Association in Tianjin.

Then in December 1936, following the Xi'an Incident, Chiang agreed to join with the Chinese Communist Party to resist Japan, a move that Matsui viewed as a personal betrayal.

[51] However, Matsui had made it clear to his superiors even before he had left Japan in August that he was determined to capture the capital city of China, Nanjing, which lay 300 kilometers west of Shanghai.

Kasahara suspects that Matsui, as an aging general with a relatively undistinguished military record, desperately wanted to crown his career with one last battlefield victory like the capture of the Chinese capital.

[53] In response to this flagrant act of insubordination, Matsui, it is claimed, made some effort to restrain Yanagawa, but he also insisted to the high command that marching on Nanjing was the right course of action.

[36][58] Matsui's command problems were made further complicated by the fact that, between December 5 and 15, he was frequently bedridden due to bouts of tuberculosis, which he had first shown symptoms of on November 4.

[65] To forestall this possibility, Matsui tacked a lengthy addendum entitled "Essentials for Assaulting Nanjing" onto the comprehensive operational orders that he passed down to all units on December 7.

[65][69] Most of the buildings and civilian homes outside Nanjing had been burned down by the Chinese Army to deprive the Japanese of shelter, so Matsui's subordinate commanders decided on their own that they had no choice but to station all their men within the city itself.

"[75] When a representative from the Japanese Foreign Ministry came to investigate the matter, Matsui admitted that some crimes had occurred and he blamed his subordinate commanders for allowing too many soldiers into the city in violation of his orders.

[80] When Matsui returned to Nanjing on February 7, 1938, for a two-day tour he assembled his subordinates, including Prince Asaka and Heisuke Yanagawa, and harangued them for failing to prevent "a number of abominable incidents within the past 50 days".

[84][85] However, the leaders of Japan's Army General Staff showed scant interest in his plan to create a new government in China and they also repeatedly refused to approve any new military campaigns under his command.

[87][88] Reports of the atrocities in Nanjing had reached the Japanese government and some within the Army General Staff blamed Matsui for mishandling the crisis and causing Japan international embarrassment.

[101] Between June and August 1943 Matsui undertook a tour of Asia, including China, Indochina, Singapore, Thailand, Burma, Malaysia, Indonesia, and the Philippines in order to promote his ideas.

Nonetheless, for his role in the Nanjing Massacre, he was convicted and sentenced to death under Count 55, charging defendants with having "deliberately and recklessly disregarded their legal duty to take adequate steps to secure the observance and prevent breaches" of the laws of war.

"[123] Shortly after hearing the verdict Matsui confided to his prison chaplain, Shinsho Hanayama, his feelings about the atrocities in Nanjing and the rebuke he delivered to his subordinates on February 7, 1938.

[126] In 1978, all seven war criminals executed by the IMTFE, including Iwane Matsui, were officially enshrined in Yasukuni Shrine in a secret ceremony conducted by head priest Nagayoshi Matsudaira.

Fifthly, he insisted that his triumphal entrance into Nanjing be held at an early date, a demand which his subordinate commanders responded to by increasing the speed and severity of their operations.

[70] The historian Keiichi Eguchi and the researcher Toshio Tanabe likewise find that Matsui bears responsibility for urging the government to march on Nanjing, which led directly to the massacre.

[130] The historian Tokushi Kasahara argues that the prosecution at the IMTFE did not attempt seriously to investigate all those who were involved in the Nanjing Massacre, and instead just decided to make Matsui the sole scapegoat for the whole atrocity.

[133] Historian Masataka Matsuura notes that the focus within current scholarship on Matsui's role in the Nanjing Massacre has distracted from the fact that his pan-Asianism was the defining characteristic of his life.