During the last sack of Orsinium, the Legions were actually helping to defend the Orcs, which means they fought the Redguards and Bretons. Were Hammerfell and High Rock part of the Empire at that time? Why didn't the Empire talk them out of the assault, and how long did the Orcs hold out?

We're so used to the post-miracle of peace status quocoon, it's sometimes hard to remember those days when the provinces were basically their own petty kingdoms who could spit in the Emperor's eye if they thought they could get away with it.

I think Hammerfell is a independent state and pretty much the legion can be used to aid one political faction against another in a armed conflict.

I'm not sure if they were in the Empire, but the Empire needs a chance to show it's Legions power once in a while to make sure people remember who's in charge.

Not when Orsinium was sacked it wasn't.

Barring any details, I like to believe it went down like this: Wayrest and Orsinium ally with each other pre-Oblivion crisis. During Oblivion crisis, King Gothryd is kidnapped into Oblivion, weakening the throne. With the Empire falling apart, Wayrest and Orsinium take advantage of Daggerfall's weakness and seize large holdings. With the rising power of a fourth influence in the Illiac, Daggerfall and Sentinel ally to fight Wayrest and Orsinium. Gortwog finally dies at the venerable age of 50, creating the power-vacuum needed for Wayrest to betray Orsinium and rally with Daggerfall and Sentinel. Wayrest has to give up some of it's gains back to Daggerfall, but gains Orsinium's old holdings (and mines). This pins the sack at somewhere around 4E25.

This theory gives Elysanna loads of credit though.

ive looked into the subject, and although its pretty clear that orsinium was sacked in the early 4th era, there are very few details on it, what i could find doesnt specifically claim that the legion fought alongside the orcs, but escorted the refugees to skyrim. although a specific date is not given for the sack of orsinium, i think it was before the great war, which means high rock and hammerfell were both part of the empire. although ive been able to find out barely anything and if anyone has some more knowledge it would be appreciated!

It would seem to go somewhere after Oblivion (obviously) but before things hit rock bottom where the "empire" consists basically of Nibenay. Things are starting to unravel, Sentinel and Wayrest see their opportunity to remove an irritant, and the empire makes a gesture of defense to try to assert that the institution is still relevant and able to protect loyal territories. The Thalmor is still consolidating its power in Summerset or starting to make overtures to Valenwood, and the great war is still 120 years off.

This. I don't believe it's ever stated that the Legion clashed swords with Redguards or Bretons. Or that they attempted to stop the sack. They were but an escort for the refugees.

In the years Post Septim empire, if your province hadn't already succeeded from the empire ala Argonia, Elsweyr and Alinor, then your province was pretty much operating autonomously. Mede likely hadn't the political influence over Hammerfell and High Rock to stop the sacking of Orsinium.

The Legionnaires were sacrificing themselves to save the refugees in the chaos.