"Frontiers are an invention of men. Nature doesn't give a hoot."
Rosenthal says this quote to show how foolish the concept of frontiers is. No matter where you are (in a city or in a so-called "frontier"), nature can be deadly. After all, nature and the animals that live in it don't care what humans label an area. Rosenthal and Lieutenant Maréchal find that out the hard way when they decide to travel from Germany to Switzerland in search of safety. Rosenthal injures himself while walking in nature, which leads to good things happening for Maréchal.
"Drive fast to the sugar refinery. I shot down a French plane. If the crew are officers, invite them here for lunch."
After Captain von Rauffenstein shoots down two French officers, he does something ironic and unexpected: he invites them to lunch. von Rauffenstein initially does this to discover more about the people who he has successfully vanquished. However, because of the lunch, von Rauffenstein discovers that he has more in common with the two men that he shot down than he thought. And it is because of the lunch that, at least in von Rauffenstein's mind, he becomes friends with the two men (but especially Captain de Boeldieu).
"My men are not young, but they enjoy playing soldiers."
This quote, said by Captain von Rauffenstein to the French soldiers, is meant to scare the French soldiers and prevent them from escaping—or at least attempting to escape. In other words, von Rauffenstein says this to instill fear in his prisoners by saying that though his soldiers are still old, they are still able to perform their duties admirably.
