Having a division headquarters identified to be the higher headquarters of a specified civil support force during a catastrophic emergency would enable that force to deploy rapidly into a disaster zone during the initial phase of a crisis response. This will be supported by, and eventually subsumed by, the more robust division headquarters if the disaster so warranted. Which
I have re-written the sentence the way it looks correct to me. This is actually two sentences, not one, and the last word “which” seems to be the beginning of another sentence. You will never find a sentence in English that ends in the word “which”. Hope this makes sense.
As far as what it means it seems to be saying that during a catastrophe there would be a division of a civil support to be the first on the scene of a disaster. Their job would be to be the first ones on the scene, and then if the catastrophe so warranted (if it was bad enough) the other parts of the division would come and take over and regain control of the situation.