A Franco/Canadian secret mission is an unusual twist for a WWII story. This and the setting of the first part of "Allied" reminded me of "The English Patient". Canadian Max (Pitt) is sent to Casablanca for a dangerous mission. Marianne (Cotillard) is the French agent already in place to help him.
Out of the desert and in London, "Allied" moves into a different territory, albeit still with plenty of style. Max and Marianne's wartime romance in exotic settings turns into a real family, but doubts arise about Marianne's identity.
London during the war as the main setting for two thirds of the movie looked very realistic. I did not mind what could be the historical inaccuracy of the Blitz, because the bombing added a layer of drama to the story.
I particularly liked the scene during the party, with Sing, Sing Sing playing in the background. It is a slightly menacing tune and it complemented perfectly the atmosphere of tension, suspicion and slight desperation.
Even if I have never been a Brad Pitt fan, he did a good job playing quiet Max, a man of a few words who sees his new world disintegrating. One can easily imagine him as a long-term bachelor falling for the beautiful, brave French partisan. Cotillard was also convincing as the ambiguous "femme fatale". Contrary to what some reviewers wrote, plenty of chemistry between the two, but also tenderness.
If you like movies with a solid plot, linear storytelling, believable characters, difficult choices, great costumes and soundtrack, then you should like this.
P.S. couple of remarks: Pitt's French was not the best and no way he could have passed for Parisian, but that did not bother me too much. The final scene is a couple of minutes too long, but again, not a major problem. However, what a relief to follow a good plot without the zig- zagging in time, overused but often useless editing style.