We read in Daniel 5:19: And for the majesty that he gave him [Nebuchadnezzar, circa 562 B.C.], all people, nations and languages, trembled and feared before him: whom he would he slew; and whom he would he kept alive; and whom he would he set up; and whom he would he put down.
And in Daniel 6:14-15: Then the king [Darius, circa 537 B.C.], when he heard these words, was sore displeased with himself, and set his heart on Daniel to deliver him: and he laboured till the going down of the sun to deliver him. Then these men assembled unto the king, and said unto the king, Know, O king, that the law of the Medes and Persians is, That no decree nor statute which the king establisheth may be changed.
Though separated by just a couple of pages in the Bible, the kings and kingdoms mentioned above were radically different. As Daniel spoke to king Belshazzar in the first passage, he hearkened back to Belshazzar’s grandfather, King Nebuchadnezzar. He reminded Belshazzar that Nebuchadnezzar was autocratic and unchallengeable. Whatever he said was regarded as law simply because he said it. Written law was subservient to him, not the other way around.
But not many years after, Babylon fell to the Medes and the Persians. And when king Darius took the throne, Daniel found himself elevated to a position of great power in the kingdom. This drew the ire of some very jealous politicians, and they tricked the king into signing a law that they knew would get Daniel into trouble. Barring a miracle it would, in fact, cost him his life.
Darius trusted and valued Daniel. When he realized what had happened and how he had been duped, he “set his heart on Daniel to deliver him: and he labored till the going down of the sun to deliver him.” But by nightfall the conspirators assembled and reminded the king that everyone was subject to the written law, including him.