> To conquer the world these days, you'd have to have an anti-ICBM defense system that can stop thousands of nukes coming at you in every direction

Or you'd just need to do what the US did in the Cold War to become—for a time—the sole superpower: run a global race and have the opposition spend themselves into collapse competing with you, and swoop in and pick up the pieces; you probably need a couple cycles of that to establish durable de facto control, but if you've got the population and resources and aren't just leveraging an advantaged industrial position that the competition will close over time at the outset, being able to do multiple cycles while growing an advantage isn't implausible.