Scalzi takes his time building up a credible world before he starts piling on outrageous details. Once the stage is set (with the explanation of how this old people's army works), he can therefore dive directly into the action and assume that the reader will know and understand enough to accept whatever Scalzi will serve.

And serve he does! The story develops from a face-value beginning that might make pacifists squirm a little in its glorification of war (without even getting especially patriotic; it's just the plain old war-as-an-adventure trope) into a growing discomfort with the way this specific war develops.

Along the way, new characters are encountered and new details about the universe learned. He will develop all of this in later books, but for now the story is just barely coherent enough to wrap up satisfyingly with a strong feeling that this is not the end of the tale.

What makes it such a great read is the mixture of ridiculously fun ideas about the technology of the future and the scientific background that makes it somehow believable. It is clear that Scalzi has done his research and still has some more inventions to follow it up. It's almost impossible to end this book without immediately planning to read the next one.