Does Galadriel's phial help win the Siege of Gondor?

Question

The battle of Battle of the Pelennor Fields happens at the exact time Sam rescues Frodo from Cirith Ungol on March 15. In "The Land of Shadow," Sam says:

‘If only the Lady could see us or hear us, I’d say to her: "Your Ladyship, all we want is light and water: just clean water and plain daylight, better than any jewels, begging your pardon."’

They keep walking and a few moments later:

‘Look at it, Mr. Frodo!’ said Sam. ‘Look at it! The wind’s changed. Something’s happening. He’s not having it all his own way. His darkness is breaking up out in the world there. I wish I could see what is going on!’

It was the morning of the fifteenth of March, and over the Vale of Anduin the Sun was rising above the eastern shadow, and the south-west wind was blowing. Théoden lay dying on the Pelennor Fields.

Sam was in possession of the Phial of Galadriel, which has powers of light, while making his wish. This lines up at the turning point of the Siege of Gondor, where The Darkness fails, which filled the hearts of men with dread, and only Gandalf and the Prince Imrahil could counter it momentarily. In the end of "The Ride of the Rohirrim" and the start of "The Battle of the Pelennor Fields" we see:

For morning came, morning and a wind from the sea; and darkness was removed, and the hosts of Mordor wailed, and terror took them, and they fled, and died, and the hoofs of wrath rode over them.

The darkness was breaking too soon, before the date that his Master had set for it: fortune had betrayed him for the moment, and the world had turned against him; victory was slipping from his grasp even as he stretched out his hand to seize it.

Does Tolkien ever explain why The Darkness fails?

The books leads me to conclude that it was Galadriel's power that dispels The Darkness and allows the victory of the Siege of Gondor.

Edit: Ok people have been getting tunnel vision on "Well actually Galadriel actually did not created..." instead of the more important question of why The Darkness goes away and if Tolkien left any clues. Sam and the Phial are the only ones shown that are related to The Darkness going away, this is why I put the theory forward.

Answer 1

On the 14th March, in roughly late afternoon/early evening, Ghân-buri-Ghân noticed the change in the air. This happened before a passage (in Book V, Chapter 5, "The Ride of the Rohirrim") that said "it was night". The Tale of Years (i.e. Appendix B) gives the 14th as the date when the Woses guided the Rohirrim along the Stonewain Valley. This started just after dawn and was done by "late afternoon", at which point Ghân-buri-Ghân made his observation, before leaving. The Rohirrim subsequently walked/rode through the night to get to the Rammas before dawn of the 15th.

Also, "sea-crafty" men on Aragorn's captured boats felt the wind shift around midnight of the 14th/15th (recounted later, in Book V, Chapter 9 of RotK, "The Last Debate"). The timing of the events at Cirith Ungol are less clear, but taking the 15th as the date Frodo/Sam escape, as in the Tale of Years, then Sam's lines you quote, asking for water and daylight, would be after when Ghân-buri-Ghân realised something was up, and when the sailors started making use of the wind to sail up the Anduin. In any case, Sam's lines (in Book VI, Chapter 2, "The Land of Shadow") are said after a passage that noted the sun was climbing over the dark clouds, so it is definitely after dawn, so definitely after the Ride of the Rohirrim on the Pelennor Fields (at the end of Book V, Chapter 5).

So there is no causal connection between Sam's utterance and the wind that drove back the darkness.

Answer 2

The power of the Phial of Galadriel has very little to do with Galadriel herself. The power of the light within is not hers; the lady's contribution was merely catching and preserving the light of the Silmaril carried on Eärendil's flying ship (observable from the ground as Venus, the morning and evening star). The Silmaril contains and emits, in its original pure form, the light of the Two Trees, which, when they were created by Yavanna and watered by the tears of Nienna, were the greatest creation since the original Spring of Arda, before its first marring.

In any case, active intervention from Galadriel during the Battle of the Pelennor Fields would have been essentially impossible anyway. On the same date, there was an orcish attack on Lothlórien, which she would have played the most important role in repelling. Her ring, Nenya, was primarily responsible for the preservation of Lothlórien (where even more of the Elder Days was preserved than in Rivendell), and that was the power that made Lothlórien impregnable to all the forces that Sauron might send against it, unless he regained the One Ring or traveled to lead the assault on the forest realm himself. When the orcs attacked three times (including on March 15), Galadriel led the defense against them, which would have left her no time to concentrate on events occurring far away.

Answer 3

It would seem highly unlikely that the Phial had anything to do with it - its purpose was to defeat Shelob. The light is from one of the Silmarils and we don't really knew if they had such powers. Covering an advancing orc army with darkness was a trick used many times by Sauron's old master Morgoth during the wars of the First Age. Though Morgoth also had all the Silmarils in his possession for most of the First Age, so we don't know if they had an ability to dispel such darkness. If they didn't, then neither would the Phial of Galadriel. At any rate, Sam used it quite a long distance away from Minas Tirith.

Also, if the Phial had the power to dispel darkness at larger scale, it would have dispelled the permanent darkness above Mordor too, at least temporarily over the Cirith Ungol area where Sam was. This wouldn't have gone unnoticed and Mordor would have responded more forcefully to that. Instead there's just some orc party from the local garrison arriving to investigate it. I think there's some talk of the orcs having seen some strange light (citation needed...), but no mentioning of the darkness over Cirith Ungol getting dispelled.

Apart from Rohirrim arriving with the morning light, "a wind from the sea" could rather be interpreted as divine intervention. The Valar - Manwë in particular, seem to have a habit of meddling in many battles of the Third Age, in more or less subtle ways. Manwë is the Vala of winds and air. The great eagles are his servants and unexpectedly showed up in several other battles, likely at his command.

I don't think Tolkien ever confirmed that any of these events were a result of divine intervention though (unless someone has a source?) so this part of my answer is a fair bit of speculation. He rather seemed to have preferred to hint of such things but leaving it to the reader from there.

Note the main reason for the hosts of Mordor to wail and flee is not the darkness disappearing, but the far more down-to-earth reason that they are getting massively flanked by some 10,000 man strong elite cavalry!

Out of universe, Tolkien simply liked the reinforcements to arrive with dawn, for dramatic and symbolic value. The battle of the Hornburg follows pretty much the same writing-style, building up with things going badly for the defenders, that are getting pushed back further and further, until Gandalf arrives with back-up at dawn, turning the battle.