The Jaga invasion
Bernardo's campaigns in the east cost him his life in 1567. His successor Henrique (his uncle) also lost his life fighting the feared Jaga not long after. Before Henrique left he put the son of his wife(from another husband) on the throne. After Henrique's death his stepson called Álvaro, became the new king. This was a very controversial move and there was strong resistance since he was not considered a legitimate heir to the throne. But before a prolonged civil war could take place Kongo was attacked by a alliance of the kingdom of Makoko, the Kingdom of Kongo dia Nlaza and nomadic cannibals. These peoples were the victims of Kongo's endless wars of territorial expansion and they had enough. The Kongolese called them the Jaga. Having gathered up forces across the Kwango valley and the region north east of Kongo, they attacked. It was said that thousands swept across the mountains and emerged from the forests. The Jaga completely destroyed Mbanza Mbata, the capital of the province of Mbata. And after ravaging much of the country the Jaga finally reached Mbanza Kongo. The royal forces tried to defend the city but resistance was futile. Álvaro was forced to flee to an island in the Congo and beg the Portuguese for help. Thousands were displaced and many sold into slavery. The mighty Kongo kingdom was brought to it's knees.
The Portuguese king Sebastião I of Portugal answered his pleas and send 600 soldiers under the command of Francisco Gouveia Sottomaior, governor of São Tomé. After a year and a half of heavy fighting the Jaga were chased out of the kingdom and Álvaro was back on the throne. Álvaro went on to finally conquer many of the eastern and northeastern regions and put them under Kongolese control. Portuguese assistance had saved the kingdom but at a heavy price.
Portuguese success
Sebastião I of Portugal had demanded compensation and a promise of Kongolese assistance in Portugal's colonial wars. Álvaro gave them access to the nzimbu shell "mines" around Luanda island and promised military assistance. Shell money along with libongo (standardized pieces of raffia cloth) was the main currency used in the Kongo kingdom. Giving Portugal right to the "mines" gave them similar financial power as the king. But this humiliation was short lived and after the Portuguese soldiers left Kongo, Álvaro took back control of Luanda in exchange for a joined attack on Ndongo. The Portuguese pressured by the increasing power of the Ndongo and the fact they wouldn't be able to defeat Kongo in a war, even in it's weakened state, relented.
Álvaro's alterior motive for joining the war against the Ndongo was actually to "reconquer" the region. To achieve this he deployed a large army consisting of about 20.000 soldiers but was defeated by the army of Ndongo. The Portuguese who were able to reorganize the army because of the time won by Kongo's war with Ndongo, had a lot more succes. Their new allies the Imbangala played a big part in this succes. They were said to feast on the flesh of humans and cut down every palm tree in an area to create palm wine. The brutal and highly efficient nature of the Imbangala was the deciding factor in defeating the Ndongo. So much so that around 1590 the Portuguese established a new colony and claimed the land between the Dande and Kwanza rivers which they called Angola. All of these wars led to thousands of slaves being sold from the ports of Angola. Which made the Portuguese incredibly wealthy. For the new colony to survive the Portuguese needed to provide as many slaves for the Portuguese and Spanish colonies in Africa and South America as possible.
Threats from all sides
Around the time of the establishment of Angola. Kongo was embroiled in another civil war. Now because Álvaro had died and had appointed his son Álvaro III. This civil war was more brutal than those from the past. Both of the Álvaros were not actually legitimate heirs to the throne. And many of the nobles challenged Álvaro II. The province of Soyo was especially difficult to control. It was ruled by a dynastic family called the house of Soyo. This was unique in Kongo since most other provinces were ruled by nobles appointed by the king. The house of Soyo was different so could maintain a permanent powerbase. In the years after European contact they had grown in strength from the lucrative coastal trade. The unique position of Soyo was shown most when the Dutch reached Kongo in 1607. Soyo interacted with them in a manner highly reminiscent of an independent power. And the Dutch were confined to Soyo for the next decade.
The kingdom was tearing itself apart and Angola, confident of it's newfound military strength, started to chip away at Kongo's southern borders. Álvaro III was too occupied with staying alive to respond in a appropriate manner. Portugal proceeded to attack vassal states in the dembos region just to the south of Kongo. João Correia de Sousa the Angolan governor proceeded to make increasingly agressive demands of Kongo. One of these was to kill Pedro Afonso the duke of Mbama who he accused of helping the vassal states. In all of this chaos Álvaro III died unexpectedly. Because of threat of Portuguese invasion, the nobles decided to make the most qualified among them king. This ended up being Pedro Afonso. João Correia de Sousa was furious and demanded Pedro gave back Luanda island, surrender the entire province of Mpemba and exempt the Portuguese from any customs duties in the kingdom. These demands were unacceptable and de Sousa knew this. He was determined to end the Kongo kingdom once and for all. In the Dembos region at Kongo's southern border he gathered up a force of 30.000 and proceeded further into Kongolese territory. The Portuguese-Kongo war had begun.