In October 1978 Uganda invaded the Kagera Salient in northern Tanzania, initiating the Uganda–Tanzania War. The Ugandans met light resistance and in November President Idi Amin of Uganda announced the annexation of all Tanzanian land north of the Kagera River. The Tanzanians organised a counter-offensive later in November and successfully ejected the Ugandan forces from their country.

Relations between Tanzania and Uganda had been tense since then-Colonel Amin overthrew Ugandan President Milton Obote in 1971. Tanzanian President Julius Nyerere supported Obote and backed an unsuccessful attempt by him to regain power in 1972. Uganda also disputed its border with Tanzania, claiming that the Kagera Salient—a 1,865 square kilometre (720 sq mi) stretch of land between the official border and the Kagera River 29 kilometres (18 mi) to the south—should be ceded to Uganda. Tensions remained high through 1978, when Amin's regime started rapidly declining due to economic problems and dissent in the armed forces. In early October Ugandan forces began making small incursions into the Kagera region. On 25 October the Uganda Army launched a large attack over the border, but was repulsed by Tanzanian artillery. The Ugandans attacked again on 30 October, quickly overwhelming the small Tanzanian contingent in the region and completely occupying the Kagera Salient. Amin declared that Uganda was annexing the region, and shortly thereafter the Ugandans destroyed the only bridge over the Kagera River, easing their commanders' concerns about a Tanzanian counter-offensive. The Uganda Army pillaged the land, stealing cattle, automobiles, and personal belongings from homes. Approximately 1,500 civilians were shot and killed, and thousands more fled south.

Tanzania was caught unprepared for war but Nyerere—after being assured by his commanders of his country's military capability to react—ordered a mobilisation and instructed the Tanzania People's Defence Force (TPDF) to prepare a counter-offensive. Mozambique sent a battalion to Tanzania as a gesture of support. The Organisation of African Unity attempted to foster a diplomatic solution but Nyerere rejected attempts at mediation. Uganda Army officers concentrated on looting and ignored intelligence reports of Tanzanian plans, and thus were caught unprepared when the TPDF initiated Operation Chakaza. Most Ugandan soldiers fled in the face of artillery bombardment, and Amin soon thereafter declared that he was unconditionally withdrawing the Uganda Army from Kagera, a claim which was bitterly contested by Tanzania. Tanzanian troops used pontoons and a Bailey bridge to move heavy equipment across the river and probed the area. By January 1979 the TPDF had retaken the Kagera Salient. Nyerere did not initially intend on expanding the war beyond expelling the Ugandans from Tanzanian territory. After Amin failed to renounce his claims to Kagera and the OAU failed to condemn the Ugandan invasion, Nyerere ordered the TPDF to attack Uganda, resulting in Amin's overthrow in April. Following the conclusion of the war, the Tanzanian Government undertook efforts to rehabilitate Kagera, but social services and the standard of living for locals remained diminished for many years. The Uganda–Tanzania War is remembered in Tanzania as the Kagera War.

In 1971 Colonel Idi Amin seized power in Uganda following a military coup that overthrew President Milton Obote, precipitating a deterioration of relations with neighbouring Tanzania. Tanzanian President Julius Nyerere had close ties with Obote and had supported his socialist orientation. Amin installed himself as President of Uganda and ruled the country as a repressive dictatorship. Nyerere withheld diplomatic recognition of the new government and offered asylum to Obote and his supporters. Uganda twice launched military attacks against the Tanzanian border that year. With the approval of Nyerere, Ugandan exiles organised a small army of guerrillas, and attempted, unsuccessfully, to invade Uganda and remove Amin in 1972. Amin blamed Nyerere for backing and arming his enemies, and retaliated by bombing Tanzanian border towns. Though his commanders urged him to respond in kind, Nyerere agreed to mediation overseen by the President of Somalia, Siad Barre, resulting in the signing of the Mogadishu Agreement, which stipulated that Ugandan and Tanzanian forces had to withdraw to positions at least 10 kilometres (6.2 miles) away from the border and refrain from supporting opposition forces that targeted each other's governments. Nevertheless, relations between the two presidents remained tense; Nyerere frequently denounced Amin's regime, and Amin made repeated threats to invade Tanzania. During the same time, relations between Tanzania and Kenya grew sour, and the East African Community subsequently collapsed. Uganda also disputed its border with Tanzania, claiming that the Kagera Salient—a 1,865 square kilometre (720 sq mi) stretch of land stretch of land between the official border and the Kagera River 29 km (18 mi) to the south—should be ceded to Uganda, maintaining that the river made for a more logical border. The border had originally been negotiated by British and German colonial officials before World War I.

Meanwhile, in Uganda, Amin announced an "Economic War," in which thousands of ethnic Asians were expelled from the country and their businesses placed under the management of Africans. The reform had disastrous consequences for the economy, which were further exacerbated by a United States boycott of Ugandan coffee on account of the government's failure to respect human rights. At the same time, Amin expanded the power of the armed forces in his government, placing many soldiers in his cabinet and providing those loyal to him with patronage. Most of the beneficiaries of his actions were Muslim northerners, particularly those of Nubian and Sudanese extraction, who were increasingly recruited into the army. Amin violently purged members of southern ethnic groups from the armed forces and executed political opponents. In 1977 a split in the Uganda Army developed between supporters of Amin and soldiers loyal to the Vice-President of Uganda, Mustafa Adrisi, who held significant power in the government and wanted to purge foreigners from the military. In April 1978, Adrisi was severely injured in a suspicious car accident. When he was flown out of the country for treatment, Amin stripped him of his ministerial portfolios. He also announced the arrest of multiple police officials, and during the following month he dismissed several ministers and military officers. The shakeup strained Amin's narrow base of power in the military, which had been declining in the face of the worsening economic situation, eliminating patronage opportunities. Fearing for his personal safety and less confident in his charismatic abilities to diffuse the growing tension, Amin began withdrawing from the public sphere and conducting fewer meetings with his troops. At around the same time, he began accusing Tanzania of violating Uganda's border. In May, Amin falsely claimed that Tanzanian troops had attacked the Rakai District. He also appealed to his allies, President Gaafar Nimeiry of Sudan and leader Muammar Gaddafi of Libya to intervene and resolve the "potentially explosive" situation, claiming that Tanzanian forces were moving within 4.8 km (3 mi) of Ugandan border towns. In July, Radio Uganda erroneously declared that Obote was planning another Tanzanian-supported overthrow attempt. Nyerere ignored the allegations.

In reality, since May, Ugandan troops and military equipment had been shifted towards the border at Amin's direction and in violation of the Mogadishu Agreement. Only one Tanzanian company was near the border region, and Obote's irregular troops were in a refugee camp. In July, Brigadier Yusuf Himid of the Tanzania People's Defence Force (TPDF) warned Uganda to cease its "repeated provocations" against Tanzania. As tensions with Tanzania increased, a number of Amin's high-ranking military commanders began to advocate war with the neighbouring state. They were opposed by other Ugandan generals who argued that Uganda Army was not ready for an open conflict. Though he had desired to annex part of Tanzania for some time, Amin initially sided with the more cautious commanders.

In August Amin dispatched squads of security agents to eliminate a battalion planning to mutiny in favour of Adrisi. The battalion was tipped off to the attack, and managed to ambush and kill Amin's forces. By early October, several mutinies had reportedly broken out across Uganda. Parts of the Suicide Battalion revolted at Bondo, while other uprisings took place in Tororo, Masaka, Mbarara, Mutukula, and Kampala, where Amin was allegedly ambushed and almost killed by revolting soldiers. The revolts were defeated by troops loyal to Amin, but many mutineers managed to flee, while unrest and confusion spread in the Uganda Army.

War broke out between Uganda and Tanzania in October 1978, with several Ugandan attacks across the border culminating in the invasion of the Kagera Salient. The precise circumstances surrounding the outbreak of the war are not clear, and numerous differing accounts of the events exist. Obote wrote that the decision to invade Kagera was "a desperate measure to extricate Amin from the consequences of the failure of his own plots against his own army." Ugandan refugees claimed that several dissatisfied Ugandan officers met at Kabamba on 27 October and drafted a 12‐point petition to Amin. The document called for the end of corruption, factionalism, and favouritism towards Nubian troops; the curtailing of powers of the State Research Bureau, Amin's secret police; the reinstatement of Adrisi and former army chief of staff Isaac Lumago; the enforcement of religious tolerance; and an end of the alliance with Arab powers. According to the refugees, the President had invaded Tanzania to distract the military from this petition.