The thing is, The Great Illusion isn't wrong. Economic interdependency is a deterrent to war. It's just that its not a very good one, because the capitalism singularity is not powerful enough to overthrow the nationalism singularity[0]. Cultural ties can provide another deterrent: for example, you wouldn't imagine, say, New York and New Jersey going to war[1]. But even then, Russia and Ukraine were rather close and still are despite the former trying to genocide the latter.

The most effective deterrent to war is to put the fear of death in the people who decide whether or not to start wars. MAD theory, while technically wrong, was a good enough story that it averted several catastrophic nuclear attacks. If only because the people with their fingers on the buttons second-guessed themselves long enough for reports to come in that it was actually just the light reflecting off the ocean and not an incoming American ICBM.

[0] Yes, I mean this in the same context that one might refer to a technological singularity. Social existence today is a nested hierarchy of supra-individual social structures, of which superhuman AGI will merely be one of its subagents.

[1] Though they did come close in the 1800s over Staten Island. They call it the Empire State for a reason.