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Revolution and Bersiap

It was mid-September before news of the declaration of independence spread to the outer islands, and many Indonesians far from the capital Jakarta did not believe it. As the news spread, most Indonesians came to regard themselves as pro-Republican, and a mood of revolution swept across the country. External power had shifted; it would be weeks before Allied Forces shipping entered Indonesia (owing in part to boycotts and strikes, in Australia, on coaling, loading and manning Dutch shipping from Australia, where the Netherlands East Indies Government in exile was based). These strikes were only fully broken in July 1946. The Japanese, on the other hand, were required by the terms of the surrender to both lay down their arms and maintain order; a contradiction that some resolved by handing weapons to Japanese-trained Indonesians.

The resulting power vacuums in Java and Sumatra in the weeks following the Japanese surrender created an atmosphere of uncertainty, but also one of opportunity for the Republicans. Many pemuda joined pro-Republic struggle groups (badan perjuangan). The most disciplined were soldiers from the Japanese-formed but disbanded Giyūgun (PETA, volunteer army) and Heiho (local soldiers employed by Japanese armed forces) groups. Many groups were undisciplined, due to both the circumstances of their formation and what they perceived as revolutionary spirit. In the first weeks, Japanese troops often withdrew from urban areas to avoid confrontations.

By September 1945, control of major infrastructure installations, including railway stations and trams in Java's largest cities, had been taken over by Republican pemuda who encountered little Japanese resistance. To spread the revolutionary message, pemuda set up their own radio stations and newspapers, and graffiti proclaimed the nationalist sentiment. On most islands, struggle committees and militia were set up. Republican newspapers and journals were common in Jakarta, Yogyakarta, and Surakarta, which fostered a generation of writers known as angkatan 45 ('generation of 45') many of whom believed their work could be part of the revolution.

Republican leaders struggled to come to terms with popular sentiment; some wanted passionate armed struggle; others a more reasoned approach. Some leaders, such as the leftist Tan Malaka, spread the idea that this was a revolutionary struggle to be led and won by the Indonesian pemuda. Sukarno and Hatta, by contrast, were more interested in planning government and institutions to achieve independence through diplomacy. Pro-revolution demonstrations took place in large cities, including one in Jakarta on 19 September with over 200,000 people, which Sukarno and Hatta, fearing violence, successfully quelled.

By September 1945, many of the self-proclaimed pemuda, who were ready to die for '100% freedom', were getting impatient. It was common for ethnic 'out-groups' – Dutch internees, Eurasian, Ambonese and Chinese – and anyone considered to be a spy, to be subjected to intimidation, kidnapping, robbery, murder and organised massacres. Such attacks would continue throughout the course of the revolution, but were most present during the 1945–46 period, which is known as the Bersiap.

Estimates of the death toll of the Bersiap period vary from 3,500 to 30,000. NIOD concluded a Dutch casualty number of approximately 5,500 with possible higher numbers but not above 10,000. Estimates of the number of Indonesian fighters killed in the lead up to and during the Battle of Surabaya range from 6,300 to 15,000. The Japanese forces lost around 1,000 soldiers and the British forces registered 660 soldiers, mostly British Indians, as killed (with a similar number missing in action). The actual Dutch military was hardly involved, as it started to return to Indonesia only in March and April 1946.