The problem is that sometimes, both sides can claim victory, and no one can totally shoot down either side's reasoning.
Take the Polish-Soviet war of 1920 for instance. Poland had invaded the Ukraine with the intent to set up a buffer against Russian imperialism. The Russians managed to chase them back to Poland with the full intent of capturing and spreading communism to Western Europe.
In the end the Poles managed to fend off the Soviets and claim a successful defence of their newborn state. At the same time, the Soviets claimed a successful repulsion of the Polish invasion of Ukraine.
So... who's right?
More can be read here:
http://en.wikipedia.org/wiki/Polish-Soviet_War