Which the NCR could easily blow up with missiles and artillery, or even bomb with vertibirds.
NCR needn't invade the Legion territory, it would be too much of a waste of resources, money and manpower, sort of like an American invasion of Japan in WW2. All they need to do is activate the nukes in the divide and point them at Arizona.
Moving on to the subject of this thread, I'm going to talk about my opinions on NCR vs Legion. I personally side with NCR:
Life under NCR is much better than under the Legion, with huge amounts of the population being slaves, and women misogynistically being viewed as breeding tools. And there is zero doubt that women aren't allowed to be traders. (Rose Sharon Cassidy mentions this last bit). They don't encourage science or independent critical thinking. You're only allowed to use technologies in the way that your god, Caesar, prescribes.
There is little to no sexist or racist behaviour in the NCR, most likely due to the community's origins in Vault 15. The republic has also shown little discrimination against ghouls and mutants.
Under NCR rule the Mojave would experience safety and security under the rule of law, the Military Police has a strong presence in all NCR territories, and there is a civilian police force in core areas. Openly carrying arms, prostitution, gambling, and slavery are not permitted within NCR city limits (in specific states, not all of them).
The NCR has high living standards for its citizens, and many can find employment as farmers, travelling merchants or soldiers.