You can't go too far without mentioning the casting. The story takes place during the Napoleonic wars and the attempts of a British captain (Cary Grant) to take possession of an outsized discarded cannon before it can be seized by Napoleon. His orders take him to a Spanish guerilla, Miguel (Frank Sinatra) who has a personal aim to capture the town of Avila from the French...and the cannon would be a loud and persuasive way of doing so. While the two men grouse at each other about their differing missions and the logistics of getting a huge hulking cannon across rolling hills and across rivers to their respective targets, they also compete for the affections of Juana (Sophia Loren), a Spanish dancer and Miguel's mistress.
Grant's captain is familiar with the cannon and its capabilities, firepower and the precise means of producing its full potential without it blowing up in people's faces, whereas Miguel doesn't have a clue about what to do with it, merely with its promise of how such an ultimate weapon can be used against French battlements. But, both men need the other, and it is just as well to get it to Avila as to keep it out of French hands. And if it can be used against them...well, so much the better.One could make a point about cannon envy here as there is a lot of pointless bickering over who's the alpha male and whose aims have priority, despite the fact that neither man can accomplish their personal missions alone, and Juana is just a representation of both men's single-minded and covetous natures underneath the brio and the sense of duty. Loren is basically an ornament here, as she was in too many films that didn't know what to do with her.
Now, Grant "works", maybe—he was, after all, British, and when did he not work?—but he was never at his best in historical parts, always more comfortable as a man of his times, no matter what his age. But Sinatra as a Spanish loyalist? And Sophia Loren, as well? It was Loren's first role in English, so any burbling of lines is understandable. But Sinatra, as good as he can be in other films, is horrible here, with a terrible kind of "Jou Gringko" accent. And it's apparent that he's marking time and not going for an Oscar this time.
I've never been one for gossip, but The Pride and the Passion was very much a project of other interests for the stars than picking up a pay-check. The troika of stars seem very distracted: Grant by Loren with whom he would fall in love, and Sinatra because he only took the job to be close to Ava Gardner who was filming The Sun Also Rises in Spain. That did not go well, and Sinatra decided to go back to the States, asking Kramer to get his scenes taken care of early to do so. How this affected the finished product is hard to say, but Kramer seems to be doing his best work where the stars aren't involved, making far better use of statues and architecture and crowd set-pieces than his above-the-title players.
And that's where the movie is at its most fascinating and more succesfful, far more than the squabbling in the love-triangle which doesn't generate any real sparks, positively or negatively. But, the challenges of that big old hurking cannon and moving it through unforgiving open country and unrelenting French forces make for interesting location filming and viewing (and, in particular, one overnight hiding of the cannon in a city's least likely location) and makes The Pride and the Passion rise above the conventional issues raised among the human actors...rather odd in a war film, where the human aspects usually are of the most importance. I don't think they planned it that way.