God's heavenly army of angels and saints in linen merged with God's army the Romans and the destruction of Jerusalem in 70 A.D. if you read below you will see that God's army was always the Army he used to judge a nation.
God's army in Rev. 19:19 was the Romans. It was the time of the Gentiles. The time God gave the Roman Gentiles to destroy Jerusalem. The Beast & his army Israel was on the opposite side. God judges nations and even uses other nations to do it is abundantly clear in the Old Testament. Take the book of Jeremiah for example. In Jer. 27:4-11 we find that the king of Babylon, Nebuchadnezzar, is called “my servant” by Yahweh. And in Jer. 25: 8-11 we read:
Therefore the LORD Almighty says this: “Because you have not listened to my words, I will summon all the peoples of the north and my servant Nebuchadnezzar king of Babylon,” declares the LORD, “and I will bring them against this land and its inhabitants and against all the surrounding nations. I will completely destroy them and make them an object of horror and scorn, and an everlasting ruin. I will banish from them the sounds of joy and gladness, the voices of bride and bridegroom, the sound of millstones and the light of the lamp. This whole country will become a desolate wasteland, and these nations will serve the king of Babylon seventy years.
So there we see an evil pagan king being used of God as his servant, as an instrument of judgment against his own people.
We find the exact same thing occurring in Isaiah 10. In Is. 10:5-19 Assyria is seen God’s “rod” of judgment. Consider verses 10:5-8:
“Woe to the Assyrian, the rod of my anger,
in whose hand is the club of my wrath!
I send him against a godless nation,
I dispatch him against a people who anger me,
to seize loot and snatch plunder,
and to trample them down like mud in the streets.
But this is not what he intends,
this is not what he has in mind;
his purpose is to destroy,
to put an end to many nations.
‘Are not my commanders all kings?’ he says.”
Other texts speak to this. In Jer. 51:20-23 Babylon is referred to as God’s hammer, or war club. In Ezekiel 21 Babylon is seen as God’s sword of judgment. And in Habakkuk 1:5-6, 12, God ordains and appoints Babylon to punish Israel. Many other passages along these lines can be appealed to.