I just don't think the war network would be enough to stop an armada large enough for the emperor. Would have to be on par with the dreadnaught fleet. The dead orbit fleet was in connection with story missions in taken king I think the intercepting of the cabal signal. So there is a precedent. Maybe if there is enough salvage on Mars that hasn't been pillaged yet but I keep that for the climax. I think I read somewhere Mars is part of the nuclear program. Could pull an Armageddon/ deep impact