Until late August Cuba only had a few technical advisors in Angola, which the CIA took note of. Neto had repeatedly requested 100 Cuban instructors but it was only after careful assessment of the situation that in the end of July 1975 Cuba decided to establish four military training centres, "Centros de Instruccion Revolucionaria" (CIR) in Angola. On 25 July fifty Cuban weapons specialists were sent to Brazzaville in order to help with Soviet arms deliveries for the MPLA.
On 3 August a Cuban delegation traveled a second time to Angola to assess the situation, to draw up plans for the training programme as requested by Neto and to hand over 100,000 US dollars. Neto had complained "of the little amount of aid from socialist countries and "that the USSR detained aid to the MPLA in 1972, even though they told us that they are now helping with arms, but it's very little compared with their vast needs". Arguelles agreed with Neto as he saw the sides in Angola "clearly defined, that the FNLA and UNITA represented the international imperialist forces and the Portuguese reaction, and the MPLA represented the progressive and nationalist forces.
After the return of the delegation on 8 August the Cubans considered the options of their instructors in Angola in case of an invasion by South Africa or Zaire which would be either "guerrilla war" or withdrawal to Zambia, where Cuba proceeded to open an embassy In a memorandum of 11 August 1975 Major Raúl Diaz Argüelles to Major Raúl Castro explained the reasons for the visit and briefed on the contents of the talks. He underlined that the aggression on the part of the FNLA and of Mobutu to the MPLA and the possible development of future actions until independence in the month of November was taken into account and the awareness that "the reactionaries and the imperialists would try all possible methods to avoid having the forces of the MPLA take power". The same day Argüelles proposed a 94-man mission to Castro.On 15 August, Castro urged the USSR to increase support for the MPLA, offered to send special troops and asked for assistance. The Russians declined.
In view of the Zairian intervention in the north and the South African occupation of Ruacana-Calueque hydro-electric complex in the south, it was decided to staff the CIRs with almost 500 Cubans instead of the requested 100, which were to form about 4,800 FAPLA recruits into 16 infantry battalions, 25 mortar batteries and various anti-aircraft units in three to six months. These 500 men included 17 in a medical brigade and 284 officers. "The decision to expand the operation reflected a feeling in Havana that … there had to be enough of them to fulfil their mission as well as defend themselves in the event the operation went awry. It was nevertheless clear that …they expected it (the mission) to be short term and to last around 6 months".
The dispatch of the Cuban volunteers started 21 August and an advance party with the most urgently needed specialists used international commercial flights. Small groups continued to trickle into Luanda on such flights as well as on Cuba's aging Britannia planes and the bulk arrived after a two-week trip aboard three Cuban cargo vessels; the first one, the "Vietnam Heroico" docked at Porto Amboim on 5 October. The arrival of two Cuban ships in Angola with instructors on board was reported by the CIA and raised no alarm in Washington. The CIRs were placed in Cabinda, Benguela, Saurimo (formerly Henrique de Carvalho) and at N'Dalatando (formerly Salazar). The CIR in Cabinda accounted for almost half of the total, 191 men, while the others had 66 or 67 each. Some were posted in headquarters in Luanda or in other places throughout the country The reason for the stronger detachment in Cabinda was the perceived threat from Zaire either to Cabinda or to the Congo. By the time the training centres were fully staffed and operational on 18–20 October, unnoticed by the world, Operation Savannah was already in full swing.
In contrast to the successes in the south, where by mid October the MPLA had gained control of 12 of Angola's provinces and most urban centres, they only barely managed to keep the well equipped FNLA and its allies abreast on the northern front just east of Luanda.The FNLA was receiving arms and equipment from the U.S. via Zaire starting in the end of July and had been strengthened in September by the arrival of the Fourth and Seventh Zairian Commando Battalions. From July to November the front moved back and forth between Caxito and Quifangondo (Kifangondo). Netu asked the Soviet Union for more support which had no intention to send any staff before independence and only reluctantly sent more arms. The Cubans were busy dealing with the arrival of the contingents for the CIRs and it was only on 19 October that they paid sufficient attention to Luanda's precarious position. Realizing the threat they shut down the CIR at Salazar only 3 days after it started operating and deployed most of the recruits and Cuban instructors in Luanda.Forty instructors from the CIR Salazar were the first Cubans to become involved in the defence of Quifangondo on 23 October 1975 when they launched an unsuccessful assault on the FNLA-Zairian forces at Morro do Cal. A second group supported the MPLA on 28 October along the same defence line to the east of Kifangondo.
Yet unnoticed by the Cubans, the territory the MPLA had just gained in the south was quickly lost to the South African advances. After South African advisors and antitank weapons had helped to stop an MPLA advance on Nova Lisboa (Huambo) in early October Zulu took Rocadas by 20, Sa da Bandeira by 24 and Mocamedes by 28 October. On 2–3 November, Cuban instructors for the third time got involved in the fighting, this time 51 men from the CIR Benguela, when they unsuccessfully tried to help the FAPLA stop the Zulu advance near Catengue. This first encounter between Cubans and South Africans also lead to the first officially recognized Cuban fatalities. "Their participation led Zulu-Commander Breytenbach to conclude that his troops were 'facing the best organized and heaviest FAPLA opposition to date'".