Albert Barnes Commentary Daniel 2:37

"Thou, O king, art king of kings, unto whom the God of heaven hath given the kingdom, the power, and the strength, and the glory;" — Daniel 2:37 (ASV)

You, O King, are a king of kings - The phrase “king of kings” is a Hebraism, denoting a supreme monarch, or one who has other kings under him as tributary (Ezra 7:12; Ezekiel 26:7). As such, it is applied preeminently to the Son of God, in Revelation 17:14; Revelation 19:16. As used here, it means that Nebuchadnezzar ruled over tributary kings and princes, or that he was the most eminent of the kings of the earth.

The scepter which he swayed was, in fact, extended over many nations that were once independent kingdoms, and the title conferred on him here was not one designed to flatter the monarch but was a simple statement of what was an undoubted truth. Daniel would not withhold any title that was in accordance with reality, just as he did not withhold any communication in accordance with reality that was adapted to humble the monarch.

For the God of heaven has given you a kingdom... - At the same time that Daniel gave him a title which might in itself have contributed to the monarch's pride, he is careful to remind him that he held this title by virtue of no wisdom or power of his own. It was the true God who had conferred on him the sovereignty of these extensive realms. It was one of the designs of this vision to show him that he held his power at God's will, and that at God's pleasure He could cause it to pass away. It was the forgetfulness of this, and the pride resulting from that forgetfulness, which led to the melancholy calamity that befell this haughty monarch, as recorded in Daniel 4.