Robson's movie probably is some kind of cinematic landmark, if only for being an early portrayal of the west's failure to subdue Vietnam. Lost Command opens during the battle of Dien Bien Phu, as paratroopers arrive to relieve an embattled unit commanded by Lt. Col. Raspeguy (Quinn). The colonel is irked to learn that his superiors have sent him a unit historian, Capt, Esclavier (Delon), but the academic proves a decent soldier -- not that that helps the unit much. They're forced to surrender (to Burt "Kato" Kwouk) and are imprisoned for some time, but under Raspeguy's leadership they largely retain cohesion and morale.
Freed at last, the men return to France, where Raspeguy courts an influential widow (Michele Morgan) who might help him secure a generalship. He can help his own cause with a good showing in Algeria, where the natives are restless. He regathers much of his old unit, including an initially reluctant Esclavier but not Lt. Mahidi (George Segal), who had returned to his native Algeria after their release. In Algeria, they gradually learn that Mahidi, who had been humiliated by racist French settlers and saw a relative killed during a protest, has taken his tactical expertise to the insurgents. While he concentrates on building a guerrila army, his sister (Claudia Cardinale) smuggles bomb-making materials to terrorists in the Casbah, eventually using an infatuated Esclavier -- who doesn't learn of her family ties until later -- as dupe to get her past checkpoints. The French forces, with widely varying degrees of enthusiasm, resort to torture to break the terror network and learn the whereabouts of Mahidi's army-in-the-making. Raspeguy leads the attack to destroy Mahidi's army and earn his generalship, but will he keep his promise to Esclavier to take their old comrade alive?...
and Alain Delon and Claudia Cardinale (below)
And here's the US trailer, uploaded to YouTube by SupportingActor.