So I was just thinking about Irene and Universe One and realized that Irene has basically lost Zeref the war by casting UO. Now before you guys try and lynch me, let me explain, I assure you that I've made sure to cover my bases on this, though on the off-chance that I am wrong I will gladly take this back. This being said, I'll explain:
1. Strategic POV
One of the first things that struck me about Universe One was what a tactical blunder it was, they've gone from having overwhelming strength and numbers on all fronts to... being condensed in one place. While I know that this wasn't the main reason Irene transported everyone, it's still a massive mistake, now they are surrounded and their fodder is being wiped out by the powerhouses on Ishgar's side. Had they stayed on all four fronts Ishgar would have had to pull back due to resources and manpower and their power would have converged, with the end result being a final offensive on the FT guild by Zeref, August, Irene (I'll explain this bit later), the rest of the S12 and all of their fodder. There would be no way that Ishgar could have pulled that back.
Another small point I'd like to add is that the Spriggan's suck at defense, instead of sending out beasts like DiMaria and August to wipe out the attackers before they get close who do they send out? Historias! It's genuinely confusing, if the Spriggans have stopped messing around now then why isn't Lessio acting like an assassin and wiping out small groups and why isn't DiMaria out there massacring groups via Age Seal? Contrary to this, the Spriggans were a force to be reckoned with on offence (even when not serious), Ishgar was being pushed back on all sides but one, and the one front that was pushed back was caused by the Spriggans not working together and two notable powerhouses showed up. On the fronts where Spriggans were cooperating (North and East) they were destroying Ishgar, with the Eastern group wiping out an entire country and the Northern group wiping out and capturing two powerful guilds.
2. Circumstances
To be honest this one is hard to judge when looking at Irene but the second you look past Irene it becomes clear that Alvarez would be much better off without Universe One being cast given the circumstances at the time. What are these circumstances? Well:
- Mest making Brandish stab August and August putting her to sleep.
- August "vs" Natsu, Lucy, Mest and Happy
- Gajeel vs Bradman (or Bloodman) aftermath
So I'll start off with the first two as they are potentially the biggest mistakes that the casting Universe One made. At the time of Universe One being cast August had just cast his nuke at Natsu and co. Now this doesn't seem like it would be a big deal but it is a huge deal. Had Universe One not being cast we can say that Natsu likely would have died right there as while he partially negated August's first attack, the next attack would almost certainly have killed him. I don't think I really need to explain why this is a massive deal but I'll go ahead anyway, Natsu is (as much as I dislike him) one of the few powerhouses that Ishgar has, had August been able to kill him right then and there then Ishgar would have lost a very powerful asset and it would have been a big blow to the morale of Ishgar. On top of this Lucy would have been killed too and that would likely make Brandish return to the war a certainty as she'd have no one she owes on Ishgar's side whilst she is evidently close to August and DiMaria. So on top of Ishgar losing two big threats, Alvarez would have gained another Spriggan, a hax one too.
As for the final circumstance, this is lesser in a way but still a big point. Had Universe One not been cast then Ishgar would have lost Gajeel (another potential threat and powerhouse) and Alvarez wouldn't have lost a Spriggan for nothing. On top of that it would have been a huge morale loss for Ishgar and Fairy Tail.
Just as a disclaimer for this section, I'm not blaming this entirely on Irene as she could not have known the circumstances going on but these are still circumstances that Universe One changed and thus she does have some responsibility for them.
3. The Goal and if the trade offs were worth it
Now the goal of Universe One was technically two fold. The first goal of Universe One was to get Zeref to Fairy Heart. While this is technically achieved it is at a much greater cost, Zeref likely could've got Fairy Heart without Universe One without:
a) Sparing so many key players for Ishgar
b) Sacrificing the tactical advantage
The only trade off would have been that it would have taken longer however I don't see that as an issue as the defense of Ishgar likely wouldn't have held for another few days (at the most):
1) August and Brandish could advance from the negotiation point going straight to the FT guild.
2) The North group could crush the Ishgar forces in the North and advance, Irene could likely fly part of the way.
3) The West group could easily advance, they have Zeref and Invel.
4) They don't need to capture the South right away.
5) With Natsu, Lucy and Gajeel dead the Ishgar morale would be at a low and they would be weak due to Jellal and Gildarts (who might not even have arrived) being the only battle ready powerhouses.
6) Even if Ishgar moved it's forces to protect the guild they would likely create a soft point in the South where Alvarez could just conquer and regain DiMaria.
With the above being said I think it's obvious that the cons far outweigh the pros, the Ishgar defense likely couldn't hold out for more than a day if they all made a push at once.
The second goal of Universe One was to get Acnologia out of the way and this is the the only part where I feel Irene was successful. This being said she sacrificed all of the above in exchange for this so I'd be hesitant to say if it was worth it as we can't really say if he would have damaged Alvarez that badly, if anything he would have wrecked havoc on the Ishgar forces by killing power houses. This could actually have benefited Alvarez too as it would mean the defensive forces would be getting weaker and weaker lowering the time it would take to get to the FT guild. All in all I won't say whether this was a success or not as the consequences of Acnologia's actions are hard to predict, especially in regards to any benefits Alvarez would feel.
Last point
With all of this being said, I'm not sure whether I would put the blame on Irene or not as she had to account for something very unpredictable: Acnologia. While it is true that she started the battle with Acnologia, in which she used Universe One, it's hard to see what else she could have done as Acnologia has had a tendency to wipe out vast groups of people (e.g Tenrou, likely attempted at Tartaros) but he has also shown up in person and just wiped his target out (God Serena). If I had to say I feel she should have waited to see what Acnologia would do but it is tough due to how unpredictable he is.
Edit: I won't be replying today, this post almost came out as gibberish tbh so I'm leaving it for today.
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. It was greater for Fairy Tail to come to the Shields rather than the Shields doing the four side attack as seen with the battle of Hargeon (FT over came the odds in that battle bcs they were packing a heavy duty assault team which overwhelmed the three Spriggan). Their vanguard team was practically dealt in the same way and Jacob was taken out. God Serena was murdered by Acnologia so he was out of the game too which I'm guessing Irene felt since she prepped her attack to stop from having Acnologia from interfering anymore. Bradman was killed by Gajeel. So 8 Spriggans were out of the game as they were dead or locked up. UO however gave each Spriggan that was captured another chance to fight again which reinforced Alvarez power again. They got 5 Spriggans back (Jacob, Ajeel, DiMaria, Neinhart, and Brandish) bcs of UO, that is some heavy duty artillery put back in their arsenal again. So UO did not only get Zeref quickly to their objective but also allowed other Spriggans to escape and regain their footing in this war. UO did far greater good than damage. If you still don't believe this to be good enough, I believe the final two reason why UO was way more good than bad will more than change your mind 
and it kind of resembles their personality.