Timeline
William Blake - Milton, published c.1804-10
The first four stanzas of this poem seem to suggest England once existed as a blessed, Christian, pastoral landscape.
Jerusalem once stood where now instead are built the corrupted and corrupting "dark Satanic mills" of industrialisation. England has fallen.
But the fist two stanzas put this in the form of questions, which are answered not directly but by the desire, expressed in powerful imagery, in the second two stanzas to "not cease from mental fight...Till we have built Jerusalem/In England's green and pleasant land".
As often with Blake, what is actually expressed here is the power of ideas and ideology to capture the mind.