Why battlefield success does not by itself define victory

How should historians decide whether a country won or lost a war?

Begin with the intended political outcome, then ask whether it was achieved; the answer applies that test using an explicitly interpreted account of China's aims.

The proposed starting point is to compare the outcome with the aims for which a state went to war. Failure to achieve an aim such as conquest or forcing an opponent to change course is treated as evidence of defeat. The reply allows qualifications, including cases where another belligerent takes part of the territory the first state intended to conquer. Applied to the Sino-Vietnamese War, the answer interprets China's aim as obtaining a more compliant Vietnamese government or a change of regime. On that interpretation, the failure to secure that outcome counts as defeat. This is an argument about the war's purpose, rather than proof that the interpretation of Chinese aims is uncontested.