First of all, You have to give points for pulling off using humouristic elements as brilliantly as this, to describe something as dreadful as war. There aren't many movies that have achieved that without feeling constrained, or more or less morally repulsive. Kubrick and Coppola were masters of that section, and Veli-Matti Saikkonen doesn't come far behind with his interpretation of Veijo Meri's classic novel.
Manillaköysi has an endless list of classic one-liners, but it's still not based on cheap laughs or anything like that. The whole humouristic aspect of it comes from describing the absurdity of war, and the whole military system, by looking it with the eyes of a simple man, who's thrown into it, and who simply doesn't give a rat's ass of it all.
The tone of it isn't overly preachy or moralizing. If I would have to describe it with one word, it would be: unglamourizing. The main point of Manillaköysi is pretty much compressed in one of the most famous quotes of it: "There's nothing supernatural about war, it's just work like anything else."