This begins a new section in Isaiah - a prophecy to the nations. This first one is addressed to Babylon, the nation of Chaldeans (19) that had conquered Israel and took many of them into exile (see Daniel).
God employed the Babylonians to discipline Israel for its centuries-long rebellion against God. Yet the Babylonians were brutal. You can read about the Israelites’ mourning the Babylonians’ brutality in Psalm 137.
But God is always just. The nation that He employed to discipline Israel He will now discipline. He will do this by whistling for another nation (5), who will match Babylon cruelty for cruelty (11). They will show no mercy (16) - the Babylonians will find their babies dashed against the rocks, just as they did to Israel. This nation is the Medes (17).
Three observations. First, God is not mocked. Even the most glorious nations are at His disposal (19). The discipline and closure that God brought upon Eden, He reserves for any other nation, too.
Secondly, there is no such thing as relativism. Because in the end, God will punish the nations according to how they deviate from His own royal law. Cruelty is cruelty; ruthlessness is ruthlessness, and God cares about it all - how one nation treats another.
The second observation: true “Christian nationalism” is not some resurrected Nazism, but loving one’s nation by calling it to repent of its particular sins, to fearing this God.