I've hypothesised a number of times when the world of Equestria's experienced war or something that would be considered war except for the lack of casualties. The thing is which of these time periods would make sense for each event? 30 hours, 100 hours, 30 days, 100 days, 30 weeks, 100 weeks, 30 months, 100 months.
Note: Italics] mean the roundabout moment in history it happened, Bold means the name of the event.

Reign of Discord Discord's Downfall: Fairly self-explanatory; brings about the start of the First Age.
First Age (after 1000) First Griffon War: First contact between Equestria and Griffany leads to potent cultural clashes.
First Age (c. 7580) The Nightmare Moon Incident: As well as conflict between Nightmare Moon's expansionism and Princess Celestia's wish for stability there's also Nightmare Moon's conquering of the sheep and the counter-attack by the alliance of Equestria and Burroland; marks the end of the First Age.
Second Age (c. 790) Second Griffon War: A military tyranny occurs in Griffany that causes the empire to experience a period of major but flawed aggressive expansionism, starting with colonial clashes with Zebrica and then an attempt to seize territory from the buffalo, which in turn leads to Equestria providing logistical support that makes the griffon dictator declare war on Equestria, leading to the biggest conflict in all the millennia of Celestia's reign and the retreat of the Griffanese frontier before a second tyranny leads to a suing for peace.
Second Age (c. 802) The War of Gems: The last casualty-inflicting war so far; the diamond dogs attempt to seize territory, wealth and slave-labour from Equestria but it occurs so soon after the end of the Second Griffon War that Equestria's better prepared than anticipated.
Second Age (1000) Nightmare Moon's Return: Self-explanatory; marks the end of the Second Age.
Third Age (1 or 2) Discord's Uprising: Self-explanatory; may or may not be included depending on the full nature of the story.

If there are any other possibilities for the times before the Second Griffon War you'd like to put forward, please do but, ultimately, if Equestria gets involved then Princess Celestia must win in the end.

which in turn leads to Equestria providing logistical support

Why, though? It's not Equestria's fight. The minotaur apparently stayed out of the whole mess, so why did Equestria sacrifice its own smallfolk for something that it had no stake in?

1601002 Under the military dictatorship the Griffanese empire became speciesist. What is more Griffany had already had heavy sanctions imposed on it by Equestria for waging war with its primary trading partner, Zebrica, and the Buffalo Confederacy had an understanding with Equestria in that in the event of an invasion Equestria would provide resources but not units to aid the confederacy in liberating the territory that was taken from them in return for settling rights in the border territories.

1601202

Ah, an alliance/treaty.

Heh, poor buffalo. They either had to secede land to Equestria or Griffany. Less of two evils, I suppose.

1600866

Second Age (1000) Nightmare Moon's Return: Self-explanatory; marks the end of the Second Age.
Third Age (1 or 2)
Discord's Uprising: Self-explanatory; may or may not be included depending on the full nature of the story.

Calling these "war" is a bit of a stretch.

1601235 Exactly, but I wasn't sure what else to call them. The closest I could come up with was 'skirmish'.

1601235
Depends on who you write them.

1601403

They're in the show, it's not really a matter of who writes them.

1601516
I used the wrong word, I mean how you write them, how you write that event.

1601639

Er.... that doesn't change the fact that they're canon events, and trying to write them into anything resembling a war is a bit of a stretch. I suppose there could, theoretically, have been armies going at it offscreen, but that's pretty far-fetched to imagine.

1601789
Point... it's all a matter of what approach you take with interpreting canon events. I have Nightmare Moon and Celestia fight in a war, and I know not everyone will go for that.

1601002 I would think equestrian thought they'd be next.

1601372
"Incident", would be my choice.

1601829
It was made pretty clear there was no large scale conflict

I mean, did it just happen off-screen and went unmentioned?

1612439
That's the thing is we really don't know

1612663
In the strictest sense. It's still a completely unreasonable theory.

1612728
We don't know if it was just a one on one fight, a full scale war, or anything in between. As I said before I may do a civil war, but I know not every one will do so or even think I have the right idea. You may disagree with me, and that's fine.

second age (c. 300) the 10 minute war- brief border disputes with Zebrica, lasted only 2 and a half months ( 10 minute war is a nickname) Peace is ultimately made on the condition that both sides cease development of firearm technology.

1601002
A way to think of it is that capturing buffalo territories would give them a straight shot to Equestria.

i think the minotaur's where in some kind of war to get the rep thay have.