This is something I don't get. Joshua Graham failed to take Hoover Dam and was publicly "executed" as a result, even if he survived. So why, then, would Legate Lanius even think about retreating? And when he retreats, he talks about it only being temporary and that he would meet you in battle another day. Why would he think that? Wouldn't he know that a similar fate to Joshua Graham more than likely awaited him? Wouldn't he have instead fought to the death?

Caesar and Joshua Graham were a lot closer than Caesar and Lanius. Perhaps Lanius is more loyal - arguably - but why would Caesar treat him any differently?

Because Obsidian cares more about "the game", and adding in more gameplay opportunities, then they do about creating a consistent world.

As I like to say, if Skyrim was made by Obsidian, you could tell Alduin to not eat the world by passing a 100 speech check, despite how little it makes sense.

Besides Altadoon having no idea what he's talking about as usual. There's actually a reason Lanius retreats.

Lanius sees the Dam as a waste of Legion resources and how there are better things the Legion could be doing with itself than the Dam. The whole idea is if you convince him to retreat, he'll go back East and try to clean up house a bit and get their priorities straight and that he'll one day march West again one day.

If what this guy said ain't enough, ret con it by whacking Caesar.

Basically, there's a difference between losing and re-ordering your priorities. That's why its possible.

If you actually say "So you'll retreat?" to him, he flies into a rage and you need to kill him.

Yes, and let's not forget Lanius may become Caesar anyway. If you run the faction quests parallel to each other, Caesar will likely die in 3 of 4 endings because he is near death from his tumour just prior to Kimball's arrival to Hoover Dam. With there being no one amongst the Legion to cure his ailment,he'd likely die before the Battle for Hoover Dam ever began.

This is all theory though based on my own guessing though.