Explanation: During the US Civil War, Prussia (later Imperial Germany) sent observers along with both sides (as did several other countries) to observe this fascinating new war and potentially learn lessons from it!

To their credit, Prussia actually did take many of the lessons of the later war to heart - especially the lessons of industrialized warfare and supply as pursued by Union General Grant - and would apply them very successfully in the Franco-Prussian War. They also came away with the notion that the early Union General McClellan was an idiot who squandered opportunities to seize the initiative (true) and that Americans, as a whole, were terribly unprofessional about this war business (also true?)