While this certainly doesn't deserve to be included in Michael Medved's "50 Worst Films Of All Time" book, it's nonetheless a disappointment when considering the talent involved!
An unconvincingly made-up Richard Burton is a good Trotsky (even if director Losey had originally wanted Dirk Bogarde); the film takes pains to depict the family-man (embittered by the Stalinists' extermination of his children) as well as the politician. Though struggling with the often unwieldy English dialogue, Alain Delon is ideally cast as the slick but icy and enigmatic assassin; still, his final break-down comes across as absurd more than anything else. However, the feminine roles in the film result in being no more than perfunctory: Romy Schneider carries on a tedious romance with Delon (they were once lovers in real-life), while Valentina Cortese appears as Trotsky's dowdy wife. Also notable in the cast is Giorgio Albertazzi as the police inspector investigating an earlier attempt on Trotsky's life; Luis Bunuel regular Claudio Brook appears unbilled in one scene as Delon's 'contact man' in Mexico.
The subject matter, in itself, isn't exactly appetizing but some of Burton's speeches are undeniably compelling (one of which mentions that Trotsky feared he'd succumb to a brain hemorrhage the uncanny irony is that Burton himself died in that manner, and at approximately the same age as the Russian leader!) and the interaction between him and Delon towards the end generates a reasonable amount of tension (culminating in Trotsky's bloody and protracted assassination). Still, at the end of the day, Losey's treatment of events is surprisingly lifeless (especially for a Hollywood exile from the anti-Communist days!) and of a seriousness which is oppressive (including the obvious use of symbolism via a gory bullfight sequence).
P.S. Incredibly enough, the afore-mentioned Bunuel (my personal favorite film-maker) once spent a night in a Mexican jail with none other than Trotsky's real-life killer as his cell-mate!