This touching but slightly enigmatic lyric is one of Robert Graves’s final poems, written not long before the dementia twilight of his last years, and haunted by the consciousness that his powers and memories were slipping away from him, like a handful of dry sand trickling grain by grain between the fingers, however tight the grasp that tries to retain them.
It presents, at least for me, some challenges of interpretation. The first stanza is perfectly clear, but then things become more difficult. ‘Grappling a monster never seen before’ – this sounds to me like a scene from Greek myth, on which Graves was an authority, but I can’t quite place it. Echoes maybe of Nike, sometimes seen as an aspect of Athene, helping Zeus in his battle with the Titans? Or even of the Indian demon-slaying goddess Durga? Anyway, I take it to be an image of his Muse, representing beauty, order and clarity, struggling against the chaos threatening to overwhelm his mind, while Graves himself, rather pathetically, not only feels guilt at the situation but feels that she is holding him accountable.
And then I find the last line problematic. ‘Stealing green leaves to light an alien fire’. What’s this about? If you want to start a fire, green leaves do not seem a very good choice of material, but setting that aside, is this Graves’s way of saying that he has always been true to his poetic faith, where the ‘green leaves’ represent poetry – of protesting that he has never ignored the Muse’s calls upon him nor betrayed her by using his gift in the service of anything else, the ‘alien fire’ representing as it were an offering to some other divinity? (Incidentally, as far as the green leaves go, I am minded here of a beautiful image in R.S.Thomas’s poem ‘Prayer’: ‘the tree of poetry/that is eternity wearing/the green leaves of time’).
Anyway, an intriguing and moving poem, and if anyone has any better ideas about its imagery I should be glad to hear them.
At The Gate
Where are poems? Why do I now write none?
This can mean no lack of pens, nor lack of love,
But need perhaps of an increased magic –
Where have my ancient powers suddenly gone?
Tonight I caught a glimpse of her at the gate
Grappling a monster never seen before,
And jerking back its head. Had I come too late?
Her eyes blazed fire and I could look no more.
What could she hold against me? Never yet
Had I lied to her or thwarted her desire,
Rejecting prayers that I could never forget,
Stealing green leaves to light an alien fire.
Robert Graves
David Sutton 
In the last stanza, it’s a bit ambiguous who is doing the “rejecting” and “stealing.” Grammatically, it should be the poet. But it’s usually the poet who does the praying and the Muse who does the rejecting. So if the Muse is the subject, she is rejecting the poet’s prayers and stealing his fertile pages (green leaves) to fire the imagination of some other artist. Maybe?
Ah, I never thought of that. The grammar, as you say, is against it, but the sense might seem to favour it.
I got the impression that the prayers were the ones learned in youth, and never wholly, regrettably, superseded (?) by his muse-obsession. He had not been able to wholly overthrow his Xian upbringing ?
As for the rest it may click into place in time as we think about it some more.