Most of the websites I looked up- just from simple google searches- agreed that America was surpassing everyone in Europe in terms of production before WWI even started. America also became a world power in the 1870s or so- something that most every one of my sources agreed upon.

Pre-WW1 USA was certainly 'getting there,' but it was also already fairly high up there. You also have to think of size- the US simply had more resources at their disposal than pretty much any European country, period, since the US is easily three times as big as Germany. And even pre-WWI, they were the ones that first invented armored ships, proved that they were very, very willing to go to war if needbe (our very bloody Civil War), televisions and cars, we made the first planes and first commercial versions.

The US might not be able to invade and take over other countries as easily as Germany would, but the US has far more potential there than other nations, and in this case, my character in this roleplay knows that. If push comes to shove, the US would spit out forces that may not be top notch- but they'd get the job done, and make up for their experience and skill in numbers and tactics.

I was reading accounts of German soldiers after world war I and II, and both times, they described American officers to be criminally lax, their men frighteningly casual with them, etc. etc., but that they feared America the most, because they took the battles more seriously than all the others. They made up for their informal demeanor and the like by being very much closer,
and more determined to win and fight for the troops next to them.