He's using "brevity" as a measurable quality that can be treated as the absence of length.

The disastrous consequence of a war (per the diagram) is directly proportional to the length of its goal. Brevity is inversely proportional to length.

It might be less confusing to say that the disastrous consequence of a war is directly proportional to the prolixity of its official explanation.

I don't know how I got so confused by that. Unusual for me, but it happens!