Like most sigificant words war has a number of meanings to Blake. Basically there are two kinds of war: corporeal and "War & Hunting: the Two Fountains of the River of Life" (Milton 35.2; Erdman 135)
1. In describing spiritual warfare Blake followed closely biblical guidance such as:
"Wherefore take unto you the whole armour of God, that ye may be able to withstand in the evil day, and having done all, to stand.
Stand therefore, having your loins girt about with truth, and having on the breastplate of righteousness;
And your feet shod with the preparation of the gospel of peace;" (Ephesians 6:13-15).
And this from Blake:"As the breath of the Almighty, so are the words of man to man in the great Wars of Eternity" (Damon 441, quoting Milton 30.18)
(Much of the post is owing to Damon's valuable Blake Dictionary.)
But the "'two Fountains of the River of Life' are become Fountains of bitter Death and corroding Hell, till Brotherhood is changed into a Curse" (Milton 35.2)
2. Blake lived in an era of almost continuous warfare on the part of the United Kingdom. As a young man, like many other Brits, he applauded the American War for Independence. His America a Prophecy fleshed out his strong feelings against the King's imperial actions.
He also showed great symphathy for the French Revolutionaries and wore the symbolizing red hat-- until the Reign of Terror came along; at that point he renounced all forms of War.
Of course Blake (and everyone else) equated War with imperial dynasties like "Alexanders & Caesars, the Lewis's & Fredericks" (Jerusalem plate 52) (and of course King George).
In The Monk of Charlemaine Blake in the last three verses (Jer 52, lines 82-93) very aptly contrasts the two kinds of War; he included all of the 'Churches' (from the time of Christ to the present), because he well understood, like too few of us today, how closely the Churches have been allied with the warring imperial regimes; they are in effect its propaganda arm.