"...Release the four angels who are bound at the great river Euphrates... The number of the mounted troops was two hundred million... A third of mankind was killed by the plagues of fire, smoke and sulfur... The rest of mankind that was not killed...still did not repent..." (9:13-21). The Euphrates river was on the boundary of the promised land, see: Genesis 15:18; Deuteronomy 1:7+, 11:24; and, Joshua 1:4. Jeremiah 51:59-64 seems relevant here.
Six trumpets have now called God's People but, "The rest of mankind...still did not repent..." (9:20-21).
See Exodus chapters 28 and 39 - breastplates were a part of the garments worn by priests. Do these troops consider themselves priests of their religion?
"The king will do as he pleases... He will be successful until the time of wrath is completed...he will honor a god of fortresses; a god unknown to his fathers...(he) will greatly honor those who acknowledge him. He will make them rulers over many people... Yet he will come to his end..." (Daniel 11:36-45).
"Woe to the city of blood, full of lies, full of plunder, never without victims! ...Many casualties, piles of dead, bodies without number, people stumbling over the corpses..." (Nahum chapter 3).
"...ruthless and impetuous people, who sweep across the whole earth to seize dwelling places not their own. They are a feared and dreaded people, they are a law to themselves and promote their own honor..." (Habakkuk 1:5-11).