Spoilers.

I'm on the second part of For the Republic quest, and have just been given the quest to convince the Great Khans to fight for the NCR. The problem is, I already did that for Yes Man; I went there and turned them against Caeser and they said they would fight at the dam.

Now, when I go there, there are no dialogue options to have them fight for the NCR, and I cannot finish the quest and continue the game.

Something similar for me right now doing it for the NCR pretty much got it done except there are no other dialogue option to continue and my quest arrow still points to him...
I've killed Pappa and I get a Quest Failed " Why Can't We Be Friends" when it isn't even active in my quest list... Which further checking isn't even related to the quest.

My quest journal said I completed the quest given to me from the NCR, saying that the Khans were all wiped out. I didn't kill a single one.....

For that I can guess you took the option that made them disband. At least that was my impression of those dialogue choices.

I just wish I knew which game save I had it on... As I do have a hundred something saves.

Reloaded to that part...

Three options I chose the second which lead me to my bug ... I'll try the other ones. Yup it looks like it might just be the second option that does that.

This is ridiculous. I'm sorry you guys are having the problem as well; I hope this gets resolved. Obsidian needs to release a patch for this ASAP.

Ran into this as well, that's the risk when you make complex faction structures like this, you may overlook things. I'm removing New Vegas until the game and addons (NVSE, NVSR, Wrye Flash, UNVP and more of the good stuff) have matured. Currently, the game is very prone to all sorts of questbreakers. There are also a fair share of issues with AI packages that appear to cause crashes. Most of my repeatable CTD's can be fixed by turning off the AI, turning it on again = short pause (freeze) followed by a CTD.

I even recall some AI packages in the GECK not even having conditions properly set which is probably the reason for some of the CTDs.... well... guess I'll be playing the waiting game instead of the American post apocalypse sim for now

I finished it without using the second option... After I reported my progress to Moore I get these quest failed for Caeser quests which I had no clue about....

I get the completion without even talking to Papa Khan about disbanding. It says that I wiped out the Khans. I want the Khans to show up at the final battle. This game is so good but so very frustrating.