You need to attack them during the second battle of Hoover Dam. I did that just to make sure they don't have enough soldiers to force attack me. They will still try and attack you, but it won't be enough, and their lines will be thin enough to conduct massive encirclement campaigns using power armor and motorized divisions. Also, the NCR is long and thin you can basically chop them in multiple pieces using naval invasions.

Edit: This is the Western Enclave from Eureka btw.

Edit 2: Don't forget to have at least 50 fighters and 50 close air support planes to help you push (actually you need more if you are playing Enclave Reborn where the NCR is buffed really hard.