Geography.

When World War II broke out, the world was in the middle of the technological transformation from cast-iron and brass to steel, and, more importantly, from coal to oil. In order to build a oil-and-steel economy, you need oil, coal, iron, and fresh water; to finance and transport all of that to the world, you also need access to a top-rate natural harbor.

North America's "rust belt" stretches from Lake Erie (one of the largest bodies of fresh water in the world) across West Pennsylvania (until it was mined out, the second-largest oil field in the world) to the northern Appalachians (at the time, some of the largest iron and coal mines in the world). It was also connected, both via direct rail route and via barge from the Great Lakes to the Erie Canal, to New York harbor. (You also need rubber, at first, but that mattered a lot less when, during WWII, a scientist in the US invented synthetic rubber that was just as good as natural rubber.)

This is, frankly, one of the things that World War II was about. The rising power of the United States made it clear to all the other imperial powers (and would-be imperial powers) that they needed all of those same resources within their empire. And nobody did. Germany had iron, coal, and fresh water, but no oil to speak of. Great Britain, ditto. The Ottoman Empire could have had all of the above ... except that their resources were too spread out and the transportation links to connect them all together didn't exist yet. Russia had the same problem. Most of the rest of the world either had even fewer than that, or had all of that but were technologically too far behind to exploit them before the imperial powers could try to conquer them (hence the decades-long scramble to control China and Manchuria).

If what you want is to kick-start what the sci-fi authors today call a Dieselpunk Revolution, there is no chunk of geography on Earth any better suited than mid-20th-century Michigan, Ohio, and Pennsylvania.