I haven't seen the movie, but i've studied the period. It's probably the most important period in Chinese history.
China as we know it began about 220 bce under the Qin (pronounced roughly, Ch'in) dynasty. Before this time there were numerous smaller city's and provinces spread across the area which now encompasses China. Only along the Yellow river did any organized civilizations form, beginning with the Xia, followed by the slightly larger Shang. When the Zhou (pronounced roughly joe btw) replaced the Shang as rulers, they found themselves governing an area too large for one central seat of power. Knowing they must allow provincial rulers under them they developed the concept of 'the mandate of heaven' to quell any latent loyalty to the Shangs, and prevent rebellion. Basically the concept was that the 'emporer' was the 'son of heaven' and that his actions were ordered by the universe and were therefore beyond contestation. It's similar to the philosophy of divine right of monarchy.
So broad was Zhou's lands, however, that the far provinces eventually came to govern themselves almost completely, and thus rebelled against the central seat of Zhou. This led to the warring states period, which is the setting for Hero. It was a pretty awful time, and was on the verge of destroying the entire area. One of the rulers of the time, Qin (pronounced roughly, Ch'in), did eventually unite the lands and began the first true empirial dynasty, the Qin (from which China is named).
The most important aspects of China stemming from this time period were 'the mandate of heaven', and unification. Without becoming unified it is unlikely China could have grown into the power it is today (be that for good or ill). Without the mandate it is unlikely that China would have retained such central authority against the spread of democracy. Therefore the movie is mostly, as I see it, about the need for the good of china (unification) to surpass the desires and goals of individuals or even the concept of justice itself. There are other famous movies about the 3 assassins who came for Qin, though the names escape me at the moment.
