Watching this again, it strikes me, politics aside, what a good story it is - and what an excellent script was made from that story. The basic situation is set up simply and quickly - getting an African leader out of jail - and the key characters all have strong motivations and distinct personalities: tycoon Stewart Granger wants mining concessions, drunken Richard Burton wants money, Jack Watson misses adventure, Roger Moore wants to escape the price on his head (put there by the mafia), Richard Harris wants to help who he thinks is a decent man, Hardy Kruger wants to get home to South Africa.
From then on the action develops logically and at a fast pace - there's training sequences, landing in Africa, a relatively easy achieving of the first goal, then - in a great twist - Granger betrays the mercenaries and they have to get home the hard way.
Characters have emotional connections to the mission - Harris is motivated by ideology, but also wants to look after his son; he asks Burton to be godfather, and Burton later has to shoot Harris; Burton, Harris and Moore are all old friends; Kruger and the African president form a friendship; Burton and Watson are old friends, to the great worry of the latter's wife. This means that when characters die it means something - I get a lump in my throat when Kurger dies, when Burton has to shoot Harris, when Burton sees Watson die.
Reginald Rose wrote two other filmed scripts for producer Euan Lloyd, neither of which was that good - The Sea Wolves and Who Dares Wins - the latter was especially poor. So I wonder what the extent of his contribution was - maybe he was very reliant on source material. Or maybe other factors intruded on the later films, who knows?
The action is flabby, Africa doesn't look particularly pretty (a lot of scrub and dirt), there are uncomfortable political overtones - while there are two well rounded black characters, and the whites are certainly nasty (mafia heroin dealers, treacherous mining magnates) the whites are always in charge: indeed, even the local Africans have to be led by Russian tacticians.
But it packs an emotional punch and has a very strong cast - Burton is perfectly cast as a washed up alcoholic, Roger Moore has some genuinely effective moments, and Richard Harris is really good as the idealistic mercenary.