Starting with rangers isn't bad, but it's best to take them out before they merge with NCR, because they just become too strong.
Basically what I did on my playthrough was focusing as much as I can on making PA and making 10 w divisions and always attack when they fight the legion
Keep the frfontlie focused. I was going for the north part first to close it down near Navarro and keep mocing south until they surrender
And, idk if you saw it, but there is a focus to access decisions to scavenge equipment after you start the war with NCR. It can provide you with a lot of equipment you will need. Also when you take strategic regions like Sac City or Shady Sands you get a bit of manpower.
If manpower is the problem you could also go fo the +1000 choice at the start
I'd suggest ignoring regular infantry and going for PA: they suffer a lot less casualties and shouldn't bee too hard to equip

how do you attack them early?
desert rangers i mean

there is a focus for that
I think it's called "The Ranger Question" in the bottom right of the tree
You have to be neighbouring them, so It means you must have cleared the Yakuza first

how many pa do you build before attacking?

My strategy:

1. Build infrastructure first. This will give you extra resources when you need them later on, and also mean your factories in the state build faster.
2. Exploit the 'Hide Activities from NCR' focus failure early on. You can do this three times, and each time it will nerf your resource extraction/construction speed/division attack for 90 days. Always do resource extraction first, since you don't need resources early on. This means you won't have to spend PP early on, when it is most valuable.
3. Near the start of the game, choose the options that give you +2000 manpower and +5% recruitable pop. If you pick the +2 PA divisions and -500 MP, you are extremely short sighted and deserve to lose.
4. Only build PA divisions - equipment is expendable; men are not. Get rid of those robot divisions as they are crap, and drain MP in battle.
5. Don't rush in and annex every nation next to you right away: wait for them to build up some industry via focusses.
6. When you want to go to war with NCR, choose the 'Poison the Bear' focus 39 days before you get the event 'The Enclave rumours are true'. This will make it so that you get the extra war support from fighting a defensive war, rather than the negative war support from an offensive war. Also wait until the Rangers are inevitably called in by NCR.
7. Build loads of divisions. You'll only need 7-800 MP in reserve if you have field hospitals and logistics in your divisions. You'll need AT LEAST 36 divisions, in 24 and 12 groups.
8. Promote generals to field marshals and give them those bonus traits.

how do i build that many divisions

Originally posted by HoRuS:

how do i build that many divisions

Hmm, how many do you end up having? What’s the biggest limiting factor for you? MP? Equipment?

@HoRuS Just follow my strategy listed above and you should have the MP and equipment to build around 50 PA divisions by the time you go to war with NCR.
If you want to stand a chance fighting NCR and the Rangers alone, you'll need them.

Thank you guys so much this is all really useful

Oh, one more thing - encirclement. When you fight as the Enclave you'll need to do a LOT of micromanaging with your attacks, but seeing how powerful each individual Enclave unit is, using encirclement tactics is very easy, and you can decimate your opponent's divisions.

i cant build enough eqipment manpower has never been an issue

Originally posted by Involved:

Originally posted by HoRuS:

how do i build that many divisions

Hmm, how many do you end up having? What’s the biggest limiting factor for you? MP? Equipment?

usually 12 pa divs

Originally posted by HoRuS:

i cant build enough eqipment manpower has never been an issue

Originally posted by Involved:

Hmm, how many do you end up having? What’s the biggest limiting factor for you? MP? Equipment?

Well, I guess you could try focus on it early on and such. There are a few focuses and decissions made specifically to aquire it. I'd recommend not making infantry equipment, or making it on as little factories as possible, same for support and alike, and focus on PA

I managed to take out the NCR with about 24 divisions, mostly PA but also a few auxiliaries, some motorized and a few robots, but they were also at war with the Legion and eventually (after I declared war) the Brotherhood, nevertheless I did most of the work.

I tend to turtle it for a little while, holding off on war with the NCR for as long as I can. Taking out the Rangers first is incredibly important as otherwise youre totally ♥♥♥♥♥♥ by a war on two fronts where both of them have more than you. I tend to keep an eye on them in the early game and wait till I see what path they take, If they dont take Hell Razor as leader then you dont need to worry about them much as they wont join the NCR and you can straight up ignore them and deal with the NCR first if you want (That said, I find its wiser to have their territory going into the NCR war as it puts you right next to Shady Sands and gives the NCR 2 borders to defend instead of a single deathstack). Normally Ill try and have enough divisions to man the border with the Rangers and then have a seperate force to para into their cities. Radar stations are pretty important for the Enclave since it'll let you see where to paradrop without it being suicide. Target all the cities deep into their territory and dont worry about the ones right next to the border. If you have your guys lined up to the border before you attack most of the time the AI will send 90% of their men to defend it leaving their core land relatively unguarded. When you drop into cities your plan should be to end the war in about a week with your drops taking the brunt of the capitulation effort and your border force stopping you being steamrolled and keeping their main force occupied. After that you just build up enough men to hold off the ncr border whilst you either do the same thing as the rangers with the cities or use paras to encircle their armies defending the borders. Once they've dropped behind their armies focus an all or nothing push on one tile to link supply with them and you've just encircled the majority of the NCR's army and are free to roll over them. A lot of people suggest waiting till the legion war to attack them which is also a good idea but I find waiting till they're on your border easier personally. Just remember that your top priority after you have the divisions should go into putting as many outposts and bunkers along that border as is possible. The NCR will end up having stacks of at least 5 or 6 divisions per tile in some cases and you cant really afford to match that in raw strength so play it safe and fortify it. Defensive focused General perks on the border armies can also help but its not a top priority.