Wikipedia Commons Illustrations to Milton's Paradise Lost Butts Set, Illustration 1 Satan Arousing the Rebel Angels
Milton chose to begin his epic in the midst of action. Satan, one of the heavenly hosts, had already rebelled against God. He had attracted as his followers a third of the angels into a battle against the remaining angels loyal to God. Blake's first illustration pictures Satan and his army after they have been defeated and expelled from heaven. The angels, not being mortal, were alive in hell and suffering regrets. Satan had not accepted defeat but was rousing his troops to further mischief.
When Milton wrote, he was not thinking only of God and the angels who opposed his leadership but of Cromwell (in whose government Milton had served) and the opposing military and political forces which brought down his government. When Blake illustrated, he was thinking also of events in his own times. The American and French revolutions had upended the prevailing order. Napoleon had led his military forces against the governments of Europe and prevailed.
The rebellion of Satan and the war in heaven are mentioned in the Book of Revelation. However as they appear in Paradise Lost, they were invented by Milton as explanations of the entry of evil into the world. In Blake's myth it was the disobedience of Urizen which initiated the appearance of Satan and the fall from heaven of a portion of the starry host.
 And there appeared a great wonder in heaven; a woman clothed with the sun, and the moon under her feet, and upon her head a crown of twelve stars:
 And she being with child cried, travailing in birth, and pained to be delivered.
 And there appeared another wonder in heaven; and behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads.
 And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born.
 And she brought forth a man child, who was to rule all nations with a rod of iron: and her child was caught up unto God, and to his throne.
 And the woman fled into the wilderness, where she hath a place prepared of God, that they should feed her there a thousand two hundred and threescore days.
 And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels,
 And prevailed not; neither was their place found any more in heaven.
 And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.
Book 1, lines 81-127
"To whom the arch-enemy,
And thence in Heaven called Satan, with bold words
Breaking the horrid silence thus began.
If thou beest he; But oh how fallen! how changed
From him, who in the happy realms of light
Clothed with transcendent brightness didst outshine
Myriads though bright: If he whom mutual league,
United thoughts and counsels, equal hope
And hazard in the glorious enterprise,
Joined with me once, now misery hath joined
In equal ruin: into what pit thou seest
From what heighth fallen, so much the stronger proved
He with his thunder: and till then who knew
The force of those dire arms? yet not for those,
Nor what the Potent Victor in his rage
Can else inflict, do I repent or change,
Though changed in outward luster; that fixed mind
And high disdain, from sense of injured merit,
That with the mightiest raised me to contend,
And to the fierce contention brought along
Innumerable force of spirits armed
That durst dislike his reign, and me preferring,
His utmost power with adverse power opposed
In dubious battle on the plains of Heaven,
And shook his throne. What though the field be lost?
All is not lost; the unconquerable will,
And study of revenge, immortal hate,
And courage never to submit or yield:
And what is else not to be overcome?
That glory never shall his wrath or might
Extort from me. To bow and sue for grace
With suppliant knee, and deify his power,
Who from the terror of this arm so late
Doubted his empire, that were low indeed,
That were an ignominy and shame beneath
This downfall; since by fate the strength of gods
And this empyreal substance cannot fail,
Since through experience of this great event
In arms not worse, in foresight much advanced,
We may with more successful hope resolve
To wage by force or guile eternal war
Irreconcilable, to our grand Foe,
Who now triumphs, and in the excess of joy
Sole reigning holds the tyranny of Heaven.
So spake the apostate angel, though in pain,
Vaunting aloud, but racked with deep despair:"
Four Zoas, Night V, Page 64, (E 343) [Urizen speaks] "My songs are turned to cries of Lamentation Heard on my Mountains & deep sighs under my palace roofs Because the Steeds of Urizen once swifter than the light Were kept back from my Lord & from his chariot of mercies O did I keep the horses of the day in silver pastures O I refusd the Lord of day the horses of his prince O did I close my treasuries with roofs of solid stone And darken all my Palace walls with envyings & hate O Fool to think that I could hide from his all piercing eyes The gold & silver & costly stones his holy workmanship O Fool could I forget the light that filled my bright spheres Was a reflection of his face who calld me from the deep I well remember for I heard the mild & holy voice Saying O light spring up & shine & I sprang up from the deep He gave to me a silver scepter & crownd me with a golden crown & said Go forth & guide my Son who wanders on the ocean I went not forth. I hid myself in black clouds of my wrath I calld the stars around my feet in the night of councils dark The stars threw down their spears & fled naked away We fell. I siezd thee dark Urthona In my left hand falling I siezd thee beauteous Luvah thou art faded like a flower And like a lilly is thy wife Vala witherd by winds When thou didst bear the golden cup at the immortal tables Thy children smote their fiery wings crownd with the gold of heaven Four Zoas, Night VIII, Page 101, (E 374) "Terrified & astonishd Urizen beheld the battle take a form Which he intended not a Shadowy hermaphrodite black & opake The Soldiers namd it Satan but he was yet unformd & vast Hermaphroditic it at length became hiding the Male Within as in a Tabernacle Abominable Deadly The battle howls the terrors fird rage in the work of death Enormous Works Los Contemplated inspird by the holy Spirit Los builds the Walls of Golgonooza against the stirring battle That only thro the Gates of Death they can enter to Enitharmon Raging they take the human visage & the human form Feeling the hand of Los in Golgonooza & the force Attractive of his hammers beating & the Silver looms Of Enitharmon singing lulling cadences on the wind They humanize in the fierce battle where in direful pain Troop by troop the beastial droves rend one another sounding loud The instruments of sound & troop by troop in human forms they urge PAGE 102 The dire confusion till the battle faints those that remain Return in pangs & horrible convulsions to their beastial state"