Your right, on the technical level. Europe with its never ending wars (only be intermitted by short truces) was well ahead of the U.S. in that regard. But on the tactical level Prussia, until it started its war series, wasn't known for ability to put up a fight (Yes, Frederic the so called Great, but during the Napoleonic wars its performance was rather lackluster).

So the German High Command had its time to learn how to mobilize, equip, march, motivate, supply and prepare an army for battle. And they had an established High Command. And they had an established military culture. The U.S. didn't have a large scale war in its history until then and I don't count the Mexican - American war large scale war.

To be honest, Prussia had a couple of trials before the Franco Prussian War (against Denmark and Austria Hungary), while the U.S. didn't. So it's not a (big?) surprise, they were ready for the French.

The Romans retaliated with excursions well into the third century CE. There is a battlefield in lower Saxony that is dated to 235 CE, the Harzhorn battle. There is a museum there now with a very detailed description of this site, which seems to have been a major battle, not just a skirmish, between Romans and Germanic tribes. But the Roman empire at this point did not to have the resources to occupy and romanize Germania east of the Rhine.

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sources: https://roemerschlachtamharzhorn.de/