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Related: Why aren't Grindelwald's events called the First Wizarding War? Why would the “First Wizarding War” be called this, when it clearly wasn’t the first? Especially when you have J.K. Rowling saying, as alluded to in this question What is the connection between Grindelwald and World War II?, that it was a global wizarding war. This invalidates DVK’s answer of it being the ‘first in Britain’.

My next thought was that maybe J.K. Rowling sees Voldemort’s war – and his seeking of the Elder Wand – as a continuation of Grindelwald’s war. This seems possible, I guess, but brings me back to my title question. Was the Grindelwald/Voldemort “first wizarding war” really the first war in wizarding history?

I have read all the books but none of the other material (magic creatures, quidditch and stuff like that.)

EDIT: People seem fixated on me asking about it being called first when it’s not first. That’s not what the question is about, it’s about why the war is referred to as the “First Wizarding War” at all when J.K. Rowling has clearly stated that 25 years earlier there was a massive global wizarding world war. So, basically what was “first” about this particular conflict?

My thought is its referenced that way only because we have the context of there being 2 wizarding wars where the belligerent is Voldemort.

Wars are named what they are for a reason. Many historians tend to think of there being 5-7 modern (modern being a relative term here going back to the 16-17th centuries) world wars, in terms of the number of nations involved and the amount of causalities. Yet, because of how it was fought, the technologies involved, and the new ways in which it could be covered by news, not to mention several other factors, we still consider the conflict that occurred from 1914-1918, primarily in Europe, to be World War I, not World War IV or VI.

Why was the conflict referred to as the First Wizarding War?

It wasn't.

The Harry Potter books don't contain any mention of the term "First Wizarding War". This implies that in-universe the characters don't use this nomenclature.

Where did it come from?

Out of universe, I suspect that the terms First and Second Wizarding Wars come from this passage in Order of The Phoenix (and foreshadowed in Philosophers Stone with the "Mars is bright tonight" comments).

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

EDIT

Regarding Pottermore and the origins of the term - the Wayback Machine has the Harry Potter wiki entry for "First Wizarding War" going back to 2007, significantly before Pottermore (2011). This shows that, at best, it was a fan term that was added to Pottermore, rather than an original part of JK's world design.

I'm going to take a risk here - I want to try to illustrate why I think the titles of 'First and Second Wizarding Wars' are suitable, even if the reasons can be a bit contradictory (that's not my fault is it?!)

'First and Second' vs 'Global'

As you alluded to in your question, history often names wars in ways that aren't necessarily numerical, but still make sense. (We all know what World War Three suggests; a future end of the world disaster is the usual thought, not a real-life war which actually predates what we call World War One but is still technically a world war).
So, Voldemort's two attacks of British wizarding society are referred to as The First and Second Wizarding Wars, perhaps asking the user to mentally continue 'in Britain' or 'against Voldemort.' Maybe even 'in Britain in modern times.'
There is a Wiki page for Gellert Grindelwald's dangerous actions in Europe (often called his 'revolution' in other aspects of the Wiki pages) titled the 'Global Wizarding War.' While it seems odd, perhaps wizardkind casually refer to 'The Global War,' 'The First War' and 'The Second War,' all knowing exactly what they mean even if the titles aren't perfect (just like our WWI, WWII, etc. The names are just fine and globally accepted, but can be picked apart quite rightly if you have a mind to).

Ways in which Grindelwald's actions may not have constituted a war

I know, I just argued that 'Global Wizarding War' is a good title, but now I'm going to talk about how it might not have been an actual war. The Wiki does the same, I'm not sure if JKR is more clear (I don't think she actually calls this a war either).

I'm just trying to reflect my findings with the key conclusion that, if nothing else, the names used for the Voldemort-related wars are suitable.

Despite there being a Wiki for Grindelwald's 'Global Wizarding War,' the Wiki for Gellert Grindelwald says: (Emphasis mine).

"The extent of Grindelwald's success in his revolutionary endeavours is unclear. He succeeded in creating a fortress, Nurmengard, to serve as his power base, apparently sufficiently impregnible that he was later imprisoned in it, inaccessible to his former supporters and breached only by Voldemort. But there is no evidence he succeeded in toppling any wizarding governments or made much of a dent in the Statute of Secrecy, as it remained stubbornly intact (though Muggle World War II was certainly a sufficiently chaotic and violent time to hide some serious breaches). In what few accounts of Grindelwald's revolution exist, his great power as a wizard is mentioned often, but there is less mention of his crimes. One murder is noted, that of Viktor Krum's grandfather, and presumably there were others, but this lack of details protects Grindelwald's moral ambiguity."

So, he may have had less impact than Voldemort and had, as I suggested in a comment, more of a reign of terror and less a war.
He had a fortress, but does that prove much in and of itself? He was building an army, but did he use it?

In Deathly Hallows, Dumbledore explains,

"I delayed meeting him until finally, it would have been too shameful to resist any longer. People were dying and he seemed unstoppable, and I had to do what I could."

So, people were dying, but does that suggest thousands in battles necessarily? He seemed unstoppable, but could that perhaps suggest that no other wizard bested him one-on-one?

The whole concept of Grindelwald's revolution feels more personal, and, unlike any war I can think of, it was conclusively ended in a one-on-one fight. Dumbledore was called upon, they battled, and Grindelwald lost. There is no suggestion that Dumbledore had to take down an army to reach him. This ended the war/revolution. This aspect does not sound war-like to me, and the name revolution is more frequently used in the Grindelwald article.

I found this a very interesting question and have wondered about it before. I did my best to find out more, found the answers inconclusive and almost left it there. In the end, I decided that this information leads to the idea that the names of the wars work, inspired by the comments that our historical titles are not always 100% accurate.

So, to answer your question: Was 'The First Wizarding War' really the first war? Almost certainly not, but the name works.

No, It Wasn't.

The new Crimes of Grindelwald trailer states

The wizarding and non-wizarding world have been at peace for over a century

Implying that previously there wasn't peace.

No, this wasn't mentioned in the books. But Pottermore speaks of the first war where the Order of the Phoenix was formed as The 'First Wizarding War'