In Indonesian historiography, the term Bersiap ("Get ready" or "Be prepared" in English) refers to the violent and chaotic beginning of the Indonesian National Revolution following the end of World War II in Asia. In Indonesia, the term Berdaulat ("Sovereign") is also used for this transitional period. It began after Sukarno's proclamation of Indonesian Independence on 17 August 1945 and culminated during the power vacuum between the withdrawal of Japanese occupational forces and the gradual buildup of a British military presence, before the official handover to a Dutch military presence in March 1946.

Thousands of European and Indo-European people were killed by native Indonesians. Many non-Europeans accused of anti-revolutionary or pro-Dutch sentiment also fell victim to violence, such as Chinese civilians, Japanese and Korean prisoners of war, as well as native Indonesians such as the Moluccans, Ambonese and Minahasans, along with Javanese people (especially those of Ningrat standings), Malays, Minangkabau people and Batak people of higher social and economic standing. The violence led to forced repatriation and the proliferation of a worldwide Indo-European diaspora.

Instances of wanton violence had decreased by the time British forces withdrew in 1946 and after the Dutch had rebuilt their military capacity in the region, though revolutionary and intercommunal killings continued into 1947. Meanwhile, the Indonesian revolutionary fighters were well into the process of forming a formal military and stemming violent excesses. The last troops of the former Imperial Japanese Armed Forces had been evacuated by July 1946.

The term Bersiap is derived from the battle cry and perpetual call to arms "Siap!" ("Get ready!" or "Be prepared!"), uttered by Indonesian nationalists when potential enemies of the revolution entered pro-republican areas. Originally coined by Dutch Indonesian survivors of the period, the term soon saw use in academic works in Dutch and English.

On Java, other terms aside from bersiap and berdaulat are commonly used, such as gedoran in Depok, ngeli in Banten and surrounding West Java, and gegeran and dombreng in Central Java.

In 2022, Indonesian historian Bonnie Triyana sparked controversy when he published an editorial in the Dutch newspaper NRC Handelsblad regarding a Rijksmuseum exhibit about the Indonesian National Revolution. Triyana, who was a guest curator on the exhibit, stated that the term Bersiap had a strong racist connotation in the Netherlands, and that its use invoked stereotypes of Indonesians as "primitive and uncivilized", and claimed that the team of curators agreed that the term would not be used in the exhibition. As a response, Hans Moll, head of the Federation of Dutch Indos (Federatie Indische Nederlanders, FIN), accused the Rijksmuseum of genocide denial, censorship, and falsifying history by ignoring that thousands of Dutch people had been brutally tortured, raped, and murdered by Indonesians because of their ethnicity. The museum later stated that the term would continue to be used in the exhibit and that Triyana had been expressing his personal opinion, not that of the curatorial team. Both Hans Moll of the FIN and Jeffry Pondaag of the Dutch Honorary Debts Committee Foundation filed police reports, but the police decided not to pursue charges.

Today, the term Bersiap killings is also used to avoid confusion.

On 15 August 1945, the Japanese surrendered to the Allies. As there was, for the most part, no Allied re-conquest of the western part of the Dutch East Indies (The eastern islands were already occupied by the Allies), the Japanese were still in charge on Java and Sumatra and had received specific orders to maintain the status quo until Allied forces arrived on these islands. Sukarno, Hatta, and the older leadership were hesitant to act and did not want to provoke conflict with the Japanese. Vice Admiral Tadashi Maeda, fearing volatile youth groups, and the demoralised Japanese troops, wanted a quick transfer of power to the older generation of Indonesian leaders.