9:18 AM PT -- Rob's photos of a naked Blac Chyna have been taken down for the 2nd time today. He originally posted 3 pics of Chyna naked (2 of 'em allegedly post-op), which were taken down by IG. But he put them up again soon after ... and had them taken down once more.

Rob Kardashian and Blac Chyna are at war ... she's claimed he beat her, he's claiming she's hooking up with a bunch of guys to torture him.

Chyna claims Rob beat her up and then tried intimidating her to stay quiet or else she'd incur the wrath of the Kardashian family.

Just 2 weeks ago it looked like they got back together, but now it's off the hinges. Rob claims his baby mama sent him a pic of her vagina Tuesday -- as if to entice him -- and then hooked up with a rapper named Ferrari.

Our Ferrari sources say he's been open about his fling with Chyna, so it sounds like Rob isn't dreaming this up. What's more ... Ferrari says he's been hooking up with Chyna for several months, which overlaps with the time she reconciled with Rob.

We're told the trigger for the war is all over Chyna and Ferrari.

Rob is now going wild, threatening Chyna that he'll block her from even seeing their child.