Was Firenze predicting the Battle of Hogwarts?

Question

During one of the Divination lessons with Firenze (after Trelawney has been dismissed) in Harry Potter and the Order of the Phoenix, he mentions a prediction of a war they were going to have.

Firenze says:

Mars, bringer of battle, shines brightly above us, suggesting that the fight must soon break out again.

When leaving the classroom, Ron says:

I mean, I could do with a few more details about this war we’re about to have, couldn’t you?

Though the moment where he mentions a war that is upcoming is not mentioned in the book (until Ron says it), Firenze alludes to it before that.

Is Firenze predicting the Battle of Hogwarts that will occur in Harry Potter and the Deathly Hallows? If not, what war was he talking about?

Answer 1

Firenze was likely predicting the larger war.

It’s unlikely that Firenze’s prediction was about the battle of Hogwarts specifically. As he explains to his Divination class, centaurs don’t tend to predict specific events in that way - they predict larger ‘tides’ of change or evil that may be marked in the stars.

“I, however, am here to explain the wisdom of centaurs, which is impersonal and impartial. We watch the skies for the great tides of evil or change that are sometimes marked there. It may take ten years to be sure of what we are seeing.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 27 (The Centaur and the Sneak)}}

Therefore, it’s much more likely that he was predicting the larger war against the Dark Lord than the occurrence any one specific battle. He also mentioned that the indications are that wizards were likely living through a period of calm between two wars, thus implying that his prediction is that there’ll be another war.

“Firenze pointed to the red star directly above Harry.

‘In the past decade, the indications have been that wizardkind is living through nothing more than a brief calm between two wars. Mars, bringer of battle, shines brightly above us, suggesting that the fight must soon break out again.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 27 (The Centaur and the Sneak)}}

The first war of the two he described in his prediction was presumably the war during the Dark Lord’s first rise and fall from power, ending when he attacked the Potters and lost his body.

“All right – Voldemort.’ Hagrid shuddered. ‘Don’ make me say it again. Anyway, this – this wizard, about twenty years ago now, started lookin’ fer followers. Got ’em, too – some were afraid, some just wanted a bit o’ his power, ’cause he was gettin’ himself power, all right. Dark days, Harry. Didn’t know who ter trust, didn’t dare get friendly with strange wizards or witches … Terrible things happened. He was takin’ over. ’Course, some stood up to him – an’ he killed ’em.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 4 (The Keeper of the Keys)}}

Therefore, it seems logical that the second war Firenze predicted was the Dark Lord’s second rise to power and the war that followed. After the Dark Lord appeared in the Ministry, Cornelius Fudge confirmed to the Muggle Prime Minister that they were at war.

“The point is, we’re at war, Prime Minister, and steps must be taken.’

‘At war?’ repeated the Prime Minister nervously. ‘Surely that’s a little bit of an overstatement?’

‘He Who Must Not Be Named has now been joined by those of his followers who broke out of Azkaban in January,’ said Fudge, speaking more and more rapidly, and twirling his bowler so fast that it was a lime-green blur. ‘Since they have moved into the open, they have been wreaking havoc.”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 1 (The Other Minister)}}

It seems highly likely that this second war was what Firenze had predicted, especially considering that centaurs predict large patterns rather than specific events, and that he’d already mentioned a calm between two wars, which are likely the two wars with the Dark Lord.

Answer 2

The Battle of Hogwarts was not a war; it was a battle as it's name implies. As a battle, it was but one part of a war. It was the final and most significant battle of the war, but it was not the war itself.

The war presumably is the entire conflict with Voldemort. At the time Firenze said this Voldemort had recently regained his body and was preparing for his takeover. While Voldemort had already returned several months prior to Firenze's statement, it would not really be a contradiction. At this point almost the entirety of the Wizarding World did not believe that Voldemort had returned, and there had probably been very few actual engagements at this point such that it couldn't really be called a war yet.

Alternatively, the fighting had already broken out but Firenze's claim was still correct. He said that fighting must soon break out because Mars was shining brightly. Presumably Mars would still be shining brightly when there was actual fighting going on. Firenze was merely saying that at the very least the fighting must break out soon, though it could be possible that the fighting had already started without people being aware of it. Note that he says that they could attempt to figure out a more precise timeline, but there is no mention that they did come to a more precise date:

How soon, centaurs may attempt to divine by the burning of certain herbs and leaves, by the observation of fume and flame...”

Note also the sentence immediately preceding the one you quoted:

"In the past decade, the indications have been that wizardkind is living through nothing more than a brief calm between two wars.

That matches up more or less with the timeline of Voldemort's runs of power. The first war was Voldemort's reign of terror prior to his downfall at Godric's Hollow, a little more than a decade previously. The brief calm was the bit more than a decade that Voldemort was gone, and the second war is Voldemort's new reign of terror after recovering his body.

In fact, the title of the last chapter of Order of the Phoenix is "The Second War Begins". This tells us that by the time Voldemort's return was publicized the war could be considered to have officially begun.