As far as the current literary genre is concerned, it is argumentative. In ancient times, such political essays were classified as essays. The answer to your question should be: Su Xun's The Six Kingdoms is an argumentative essay.

The Theory of Six Kingdoms is the representative work of Su Xun's political theory. This paper puts forward and demonstrates the incisive argument that the downfall of the six countries is "blamed on Qin", criticizes the humiliating policy of the Song Dynasty towards Liao and Xixia, and warns the rulers of the Northern Song Dynasty to learn the lessons of the downfall of the six countries so as to avoid repeating the same mistakes.