If it wasn’t for Blizzard’s typical contrivances, the Horde and Alliance wouldn’t have any reason to be hostile to one another. Jaina Proudmoore’s attack on Zandalar isn’t even the most pointless and egregious source of fighting between the Horde and Alliance.
Take the Worgen and Forsaken rivalry for example.
Worgen:
From the Worgen perspective, the Forsaken Queen invaded them for no reason, and then killed their prince and blew a biological agent in their capital city when she realized she was about to lose. You fled to the far corners of the earth, taking shelter with other races who rented out their worst Airbnb options to your people.
A few years later, you’re sent with your king to spy on the Forsaken, and he decides to attack, which costs the Alliance its trump card. You fight and spar with the Forsaken all pumped for revenge, while your king fights their queen. Your king smashes their queen’s new toy, and considers it a major victory.
Later, the Forsaken queen tells the Horde that because you attacked her on the Broken Isles, she and the Horde should attack your Airbnb, which burns down because it’s apparently not up to proper fire codes. You are once again homeless—double homeless.
A decade later your king and princess finally decides to return to your country to find it’s full of racist squatters who you have to throw out with the help of the Forsaken who forced you to leave. The princess is crowned as the new queen, because the king is embarrassed that he’s old and still racist.
Forsaken:
From the Forsaken perspective, Garrosh was literally about to throw your poeple out of the Horde unless you invaded an entire country you had previously left alone, so that he could use it to build a single port. After failing to hold it, you are counter-invaded by the Wolf-people who lived there, and now half of your territory is full of them. You’re flown into Gilneas to check on the viability of holding the nation, and declare it a bust when the Alliance reinforces it via the 7th Legion. As far as you understand it, the Alliance owns Gilneas and the war is over.
Surprise, the Gilneans heard you’re headed to the Broken Isles and they’re mad about it. They ambush you and do their best to cause problems, while you’re just trying to collect the Maguffins for Dalaran. You’re not sure why this is going on in the middle of the largest Burning Legion invasion, but everyone just kind of shrugs afterwards.
Your queen orders you to invade the Night Elf lands to get back at the Gilneans, and you’re ordered to hold it. You start getting flashbacks of Garrosh’s stupid port idea. You lose and retreat again, just like last time.
Eventually you’re brought in by Lillian who wants to pay a favor to her fellow Uncrowned gal-pal, who happens to be the princess of Gilneas, and Calia wants to help the Gilneans get their country back. Again, you’re not even sure why they don’t have it because it was full of 7th Legion and Gilnean regiments when you last saw it.
You fight the Scarlets alongside the Gilneans and then leave while Calia names herself queen, thinking no one would notice, so you slap her and the Blizzard quest writer, and she drops the unearned title in embarrassment.
So what was the point of all that?
The Worgen are sold something of a cartoon rivalry where they are He-man and the Forsaken are Skeletor, and the prize was an empty city after a decade of waiting.
The Forsaken aren’t given a similar grievance or rivalry, instead they’re just wondering why the Worgen keep shouting “Gilneas will rise again!” even though from their perspective, the Gilneans never actually lost it.