I think I saw this theatrically under both titles. The first time was at a movie house in Plattsburgh NY, about July 1965, and the second time was a few months later at the Wayne Theatre in the Philadelphia suburbs. By the second time, the posters proclaimed "The Great Spy Mission," with "Operation Crossbow" underneath, but the film's opening titles still called it Operation Crossbow. This Carlo Ponti production has always stood a lot of sneers for its top-billing of Ponti's wife Sophia Loren, who has only a brief role in the middle of the picture. But really, it's a pretty good flick, particularly in its production design and the casting of Tom Courtenay. I was quite young, and the atmosphere of wartime London deeply impressed me. You had a Winston Churchill actor intoning sonorously at a mahogany desk with barrage balloons hovering outside the window. When Tom Courtenay shows up to be inducted into the mission there are those wonderful Abram Games-style propaganda posters hanging on the wall-Loose Lips May Sink Ships, and all that. I was quite young and this was all new to me, however cliché these trappings might seem now. The best parts of the picture were the historically accurate depictions of the V1 project. The sex 'n' spies subplots seemed confused and perfunctory to my young eyes, just routine filler to provide a semblance of a storyline and an excuse for Sophia Loren to appear. I'm pretty sure Courtenay gets caught and killed. He was the about the only major player who didn't look as though he had wandered in from the wrong soundstage with mid-60s hair and makeup. The minor players and the sets were well done, on the other hand.
One minor oddity of the film is that it was done in color, and very good color, too. It was about the first serious, grown-up movie that wasn't in black-and-white. Two other good war films came out around the same time, 'The Train' with Burt Lancaster, and Otto Preminger's incredible 'In Harm's Way' (starring John Wayne, with Henry Fonda, Patricia Neal, Paula Prentiss, Brandon de Wilde, Burgess Meredith, Patrick O'Neal, Dana Andrews, and just about anybody else they could sign up), and these flicks were in glorious b/w. As were the more contemporary 'The Spy Who Came in from the Cold' (set in the early 60s with the first film appearance of the character George Smiley) and that ur-60s London film, 'Darling.' Serious films for grown-ups were in black-and-white up until about 1966 or 1967; color was for Hollywood comedies, musicals, and kiddie fare. A color film about the Second World War was difficult to take seriously in 1965. This, rather than the name 'Operation Crossbow,' was its real problem on first release.