Wild Geese

Allen Faulkner (Richard Burton), a British mercenary and former army colonel, arrives in London to meet the rich and ruthless merchant banker Sir Edward Matheson (Stewart Granger). The latter proposes a risky operation to rescue Julius Limbani (Winston Ntshona), the imprisoned leader of a Southern African nation who is due to be executed by General Ndofa, the man who deposed him. Limbani is currently being held in a remote prison in Zembala, guarded by a unit of Ndofa's personal troops known as the "Simbas".
Faulkner accepts the assignment and begins recruiting 49 other mercenaries for the job, including officers he had worked with on previous operations: Rafer Janders (Richard Harris), a skilled military tactician who's made a living as an art dealer, and Shawn Fynn (Roger Moore), an ex-pilot who had been working as a currency smuggler for the London mafia. Shawn also brings in penniless South African Pieter Coetzee (Hardy KrÃ¼ger), a former soldier in the South African Defence Force who wishes to return to his homeland and buy a farm. Hired soldiers include sergeants Tosh Donaldson (Ian Yule), Jock McTaggart (Ronald Fraser), Jesse Blake (John Kani), Derek Frice (Joe Cole), and a homosexual medic named Arthur Witty (Kenneth Griffith). Additional members in the group are Esposito, (Glyn Baker), Samuels (Brook Williams), Keith (Percy Herbert), Alexander (Ken Gampu), Newman, (George Lane Cooper), Finchley, (Ryno Hattingh), Gennaro (Graham Clarke), and many others. With the tacit approval of the United Kingdom's government, the hired soldiers are transported to Swaziland to be equipped and physically trained. Before the operation begins, Janders exacts a promise from Faulkner to watch over his only son Emile should he fail to return from Africa. Their celebration of Christmas Day is postponed to the 26th, after their scheduled return, instead of the 25th.
The mercenaries are then transported by hired aeroplane to Zembala and parachute in near the prison. They infiltrate the facility and rescue a live (though very sick) Limbani. The group then makes its way to a small airfield to await pickup, deeming their mission a success.
Back in London, however, Matheson cancels the extraction flight at the last moment, having secretly secured mining assets from Ndofa in exchange for Limbani. The plane takes off as soon as it has landed, without explanation. Stranded deep inside hostile territory, the abandoned mercenaries are forced to fight their way through the bush country, pursued mercilessly by Simba troopers. While crossing a river they are attacked by an aircraft which kills several mercenaries and destroys the bridge, trapping a small group on the wrong side of the river. Faulkner, Coetzee, Blake, and Witty protect Limbani while searching for another way across the river.
The mercenaries make their way towards Limbani's home village of Kalima intending to rally support for a rebellion, but they find the people are too ill-equipped to fight. At the village, an Irish missionary informs Faulkner and his surviving men to the presence of an old Douglas Dakota transport aircraft near their location, which the mercenaries may use to flee the country. As the Simba troopers close in, the group reaches the plane and stage a last stand on the airfield while Fynn attempts to get the stalled Dakota started. He is ultimately successful and the surviving mercenaries attempt to board under a hail of bullets, however in the battle most of the mercenaries including Esposito, Keith, Alexander, and Young, are killed. Janders, McTaggart, Finchley, and Gennaro stay behind to cover the group; however on the run to the Dakota, Finchley is shot and killed while McTaggart is mortally wounded, and Janders helps him and the last mercenary Gennaro, into the Dakota. Janders is wounded and left behind on the runway. Limping behind the accelerating aircraft he implores Faulkner to kill him to spare him from capture and torture, and Faulkner reluctantly complies. On the way home McTaggart dies from his wounds; Faulkner, Fynn, Donaldson, Blake, Samuels, Gennaro, and six other mercenaries are all that's left. Fynn manages to cross into nearby Kariba Airport, Rhodesia and land the aircraft, but it is too late â Limbani has died from the strain of the journey.
Several months later, Faulkner returns to London and breaks into Matheson's home, forcing him to empty all the cash in his wall safe â which amounts to half-a-million dollars. Faulkner then kills Matheson before making a swift getaway with Fynn. The film ends with Faulkner fulfilling his promise to Janders by visiting Emile at his boarding school.

Question: WHAT HAS MATHESON SECRETLY ACQUIRED?
Answer: mining assets from Ndofa.