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Alright first things first. There a thread out there for an era of quiet. These are two different timelines.
How high can a America level nation sustainably keep its military budget in the late 1800s to mid 1900s? 75%, perhaps 80%? The Great Empires have disintegrated and chaos reigns. No one from Europe will interfere because they think you're getting too powerful. Most nations in the world are just concerned with watching their own back and fending off threats from their local environment. There's a bunch of weak nations nearby whose populations you could potentially assimilate into your own nation.
The nation has no idea how long this era and campaign is going to last so the expenditure will be calibrated to last for the forseeable future (spoiler: it lasts for 30 years after which everything stabilizes with a new bunch of empires in charge).