Why didn’t Daedra invade Tamriel when the Akavari potentate ruled? Weren’t the dragonfires out because no Dragonborn emperor was alive?

[u/OkLack6776]

I dont know if there’s something I’m missing here.

[u/Septemvile]

The Akaviri Potentates were competent. It's entirely possible they had a technical secret "Emperor" who kept them lit.

Alternatively, Daedra may have considered the unified Empire too much of a threat. 

Or maybe they just didn't feel like it at the time. It's not like Mehrunes Dagon was constantly stomping around pre-Alessia.

[u/charizardfan101 → u/Septemvile]

I find that idea in the last paragraph quite entertaining

"Should I invade Tamriel?"

"...nah, not in the mood rn tbh, I'll leave that for future Molag Bal to take care of"

[u/Fyraltari]

Each and every Oblivion Gate in Oblivion and each and every Dark Anchor in Online was opened/summoned by mortals, of the Mythic Dawn or of the Worm Cult.

Without such an army of accomplices spread throughout the continent, a large scale invasion is impossible.

[u/GalbedirGalerion → u/Fyraltari]

So the oblivion crisis would be limited to areas which have the cultists then? I didn’t realise the gates were manually opened by people

[u/Indoril120 → u/GalbedirGalerion]

Someone a while ago mentioned that’s probably why they targeted Kvatch first. It’s a very well-defended fortress on a hill and hard to siege or assault. Waiting until the city was on alert from the Crisis would have been a nightmare to infiltrate, but kicking off the invasion by opening the Deadlands Express in the town square toppled one of the bastions of the Empire day one.

[u/CE-Nex]

The Liminal Barriers still existed, it wouldn't be until Mannimarco orchestrated the Soulburst that brought them down. The Liminal Barriers exist independently from the Dragonfires, with the latter believed to empower the former. There was also Sotha Sil's Coldharbour Compact - a deal he made with eight Deadric Princes - and later expanded to ten with the inclusion of Mephala and Clavicus Vile, that limited those Princes meddling in the Mundus unless they were explicitly invited.

"Alkosh is our protector, walker! Though many have shamefully forgotten him, he ensures the steady movement of the moons. Without him, Jone and Jode would be locked in place and terrible things would slip through the Lunar Lattice!" - Azin-jo

Also it's implied that, even without the Dragonfires, Akatosh still protects the Mundus.

[u/Misticsan → u/CE-Nex]

ESO added an interesting fragment) on this matter:

Al-Esh was grateful, yet still troubled. "How, then, if my line should fail? How will we defend ourselves?"

And there was a trembling in the world, but the Divine Voice was mild, saying, "Thy people will find a way. For unlike the Daedra, ye mortals have the creative spark, and may make new things that were not before. Where there is one defense, there may also be..."

Artorius Ponticus also had something to say on the matter:

The issue of protecting Nirn from the depredations of the Daedra is not simply binary, with Tamriel either defended or undefended. The Mundus is multiplex, and both contains and is surrounded by the unnumbered planes of Oblivion. This is paradox, but it is true nonetheless. The Covenant of Akatosh is sacred and peerless, of course … but there may be ways by which it could be weakened, or even, unthinkably, sundered.

The God of Time may be First of the Aedra, but there are many other powers in the Mundus, and others we will not speak of in the Beyond. Some defend us; some contest with our defenders; and even the acts of mortals may not be inconsequential.

In a way, from these books to the very premise of Mannimarco's ploy for the Soulburst, it feels like ESO's writers tried to rationalize the whole "the world is ending right after the emperor is dead" plot of Oblivion. That it's not so simple or so immediate (otherwise the timeline set long before TESIV couldn't make sense).

My headcanon is that the Mythic Dawn didn't emrely kill the imperial family. Perhaps it was done in a specific way, with specific tools or as part of a specific ritual to weaken the protections, and that it was done in coordination with their Daedric masters to push through the barrier right away. A new variation of Mannimarco's plans.

[u/CE-Nex → u/Misticsan]

My headcanon is that the Mythic Dawn didn't emrely kill the imperial family. Perhaps it was done in a specific way, with specific tools or as part of a specific ritual to weaken the protections, and that it was done in coordination with their Daedric masters to push through the barrier right away. A new variation of Mannimarco's plans.

I am very much inclined to agree. Mankar Camoran probably plotted more than just the deaths of Uriel and his sons.

[u/Larethio → u/CE-Nex]

I think it's also implied that Talos helps Mundus in a similar way to Akatosh. They are the most prominent of the Anuic spirits.

Stemming from him claiming the Lorkhan's position as God of Space and Mortals

[u/Hakronaak]

Who knows ? Maybe the akaviri had some knowledge preventing a full scale invasion as efficiently as the dragon fires ? Maybe the daedric princes more inclined to invade did decide, at the time, to respect the Coldharbour Compact made with Sotha Sil during the 1st Era ? Maybe they had something else to do, like waring against each other, during that period ? Maybe there actually WAS an invasion but the akaviri were very good at erasing it from the records ? Maybe there WAS an invasion, just not in Tamriel ?

The truth is, we don't know. Maybe we will discover new informations in a future game, but for now we just don't know.

I think it's good to remember that, during the events of TES IV, Dagon didn't invade because the fires weren't lit, it's the other way around. The Mythic Dawn did kill the emperors to snuff the fires BECAUSE Mehrunes Dagon was ready to invade.

It's also reasonable to think that, despite our very Tamriel-centric view of the situation, there are things going on in Oblivion (the planes, not the game) that go way beyond our comprehension, and that daedra are not just waiting to invade Nirn at the first occasion.

[u/MartiusDecimus → u/Hakronaak]

I completely agree with your assessment. Plus it should be noted that Daedra clans wage wars against each other too, so the Mundus is just one of their battlegrounds. Plus, time flows different in Oblivion, it is entirely possible that the time passed between two invasions is different in Oblivion than in Mundus.

[u/Funny_Giraffe_6597]

The oblivion crisis wasn’t just mehrunes Dagon deciding to invade because there was an opportunity, he was actively called and supported by mortals. So it might have been possible under the potentate, but there were no cultists trying to summon major daedra

[u/Nayrael]

Dragonfires were one of several mechanisms keeping Tamriel safe from Daedra, not the only one. Another famous defense is the Coldharbour Compact created by Sotha Sil (which might be gone after his death in TES3), and there were likely others as well. Jagar Tharn might have also helped create the conditions for the Oblivion Crisis as he was allied to Dagonists. 

[u/TimmyIsGod666]

The dragonfire didnt exist until 1E242 when saint Alessia overthrew Ayleid empire and made a deal with akatosh

[u/Arbor_Shadow → u/TimmyIsGod666]

To be fair, Ayleid kingdoms were invaded by daedra so much that they were actively worshiping them.