I thought you were suggesting that the US' low levels of defence spending wouldn't be an issue, since they could just ramp up training and production once war breaks out. You said (paraphrasing the pro-British position): "Because their defense spending was so low on day one they start out with a decisive disadvantage, and can't make it up, because any arrangements would be too makeshift to be effective." If "on day one" in this sentence doesn't refer to day one of the war, what does it refer to?
Sorry I thought I was being pretty clear. In the 19th Century the U.S. believed it could rely on a militia/national guard system to defend it's land borders. I think everyone on this board knows this. That always turned out causing serious problems when war broke out. The War of 1812, and the Civil War are notable examples of chaotic mobilizations. 1917 wasn't much better.
What triggers these hot debates are such assertions that the War of 1812 was fought to a draw. "No the U.S. clearly lost, because they didn't conquer Canada." "The war was all about American aggression against Canada, what was happening on the Atlantic was just an excuse." "The Americans needed to suck it up, because the British needed to attack it's commerce, because they were saving the world from the Tyranny of Napoleon." The U.S. only survived without losing territory, or having to accept harsh British terms because the British gave them a break. "If the war went on any longer the U.S. would've collapsed." Technical arguments are made to support that contention, and any argument that, that is counter factual is greeted with a dismissal that your only claiming American Exceptionalism.
Trent Affair debates go the same way. Union resistance would simple be impossible, because they Can't make guns, bullets, or gunpowder without the British. Any arguments that the situation would be desperate, but the Union wouldn't just give up. That there were other sources of high grade iron, nitrates, and lead, that given time would replace British sources, are dismissed as American Exceptionalism.
In this 1890 scenario any argument that the U.S. Army had a more modern tactical doctrine, or that it's equipment might be adequate to the job is again dismissed as, you guessed it, American Exceptionalism. In all these debates I've never said the U.S. would win, only that they might, or have a reasonable chance of winning, or making the war so costly that favorable terms would be negotiated. That was always going to be U.S. strategy in any 19th Century war with Britain. The other side, (not everyone, just the most strident) assert the chances of American success are slim to none. In 1890 the British have to win because their regulars. Advancing in line, for massed volley fire, saber, and lance charges, and close support guns be damned. That the U.S. Army could have better leadership at this point in history are dismissed as you guess it.
If instead of fighting the United States the British Army fought a campaign against the French in North Africa, or in Belgium, would the same people be arguing the British would clean their clocks? If they fought the Germans, say in Denmark would they argue the British clearly had the better army? I seriously doubt they would. In the Period from the 1870's till after the post 2nd Boer War reforms the British Army, it's attention absorbed by it's Colonial Wars had fallen behind it's rivals.