If they win. By the way, you're also forgetting the South Vietnamese, the Soviets, and the Chinese. It wasn't just two parties involved, you can pretty much not interfere with the course of the battle a whole lot in the Inde and House endings, and the NCR still defeats the Legion and makes it all the way to the Legates Camp (although you already killed/coerced Lanius. But well, he wouldn't have survived anyway from either the NCR or the Securitrons) the only difference from the NCR ending is that House/the Courier shows up with the Securitron Army afterwards to either kill/coerce Oliver. In three out of four endings, Caesar's little mini-Tet blows up in his face, I think that's enough evidence to conclude that approach was flawed. Small attacks everywhere by weak and unrelaible allies
I specifically stated that the South Vietnamese, Soviets, and Chinese aren't all anxiously waiting the outcome of the battle to decide the fate of their entire nation. Boomers, Khans, etc. all are.
In House's ending, he couldn't have done anything without the Courier and same with Yes Man. Saying Caesar's approach is flawed because he didn't see HUNDREDS OF SECURITRONS pouring from some unremarkable looking weather station doesn't follow. Caesar prepares for what's reasonable and in the case of House and Yes Man, House could've just as easily killed the NCR. House wants NCR as customers.
Yes Man? It's literally a clone of the House endgame. Only difference is the dialogue and the choice to blow up the Dam's generators. The endings are different but the Battle itself plays the exact same.
Small attacks? Let's say Courier helps nobody and just sits at home drinking Nuka-Cola:
Forlorn Hope falls to Legion. Camp Golf falls to Legion. Monorail to Vegas bombed by Legion. Omertas attack the Strip, even if they fail, cause a diversion and bomb the Embassy. Camp McCarran is attacked by the Fiends. Freeside breaks out into riots. Novac falls to Legion.
That's not small attacks. Caesar's Legionaries aren't all concentrated at the Dam. They take all these places while the Battle goes on. Yes, WHILE the Battle's going on. The NCR emergency radio says so.