RENAULT: Unoccupied France welcomes you to Casablanca.
Renault is pretty much the "Dig King." He gets in little barbs whenever and wherever he can. While he remains loyal to Strasser and the Reich on the surface as much as possible, we suspect (and later have it confirmed) that he isn't a complete Nazi fanboy either. Here he can't resist reminding the Nazi officer that France is currently unoccupied by Germany, under the guise of a simple greeting. How do you say "oh, snap" in German?
FERRARI: My dear Rick, when will you realize that in this world today isolationism is no longer a practical policy?
There's a theory that Rick's isolationism was symbolic for America's initial refusal to join the war effort, and it's supported by lines such as this one. It's unlikely Ferrari's reprimand had any effect on Rick's eventual decision to man up, but maybe it got him thinking.
RENAULT: A very clever tactical retreat, Major.
This isn't in reference to an actual military retreat, but more playful prodding from Renault, who compliments Strasser on his diplomatic handling of his meeting with Laszlo by using the sort of language he feels a Nazi officer might be able to appreciate.
RICK: Nothing can stop them now. Wednesday, Thursday at the latest, they'll be in Paris.
Because there are currently no German troops in Casablanca (Strasser's boys don't really count), the flashback is really the film's best opportunity to give the audience a sense of the impending doom of military invasion. We can understand Rick and Ilsa's desperation as the Nazis close in on Paris, as their paradise starts to crumble. Note to John Milton: Paradise Crumbling would have been a better title.
ILSA: Was that cannon fire, or is it my heart pounding?
Here's another line that comes during the flashback, as Ilsa can hardly distinguish the pounding of cannon fire from the pounding of her heart. It's props to this movie that she can get away with this kind of melodramatic dialogue without the audience starting to leave the theater.
RICK: Don't you sometimes wonder if it's worth all this? I mean, what you're fighting for.
LASZLO: You might as well question why we breathe. If you stop breathing, we'll die. If we stop fighting our enemies, the world will die.
With all the indirect references to war, here's a flat-out statement of what's at stake, according to Laszlo—the end of the free world as we know it. Everyone knew in 1941 that the Germans would overrun Europe and Great Britain if the U.S. didn't get involved. This was probably why Laszlo was so insistent on getting there. By 1941, the U.S. was supplying weapons, but people like Laszlo knew that the whole "Arsenal of Democracy" approach wasn't cutting it.