Did they not know that German armies were streaming across Europe towards the English Channel?
Did they not know about the treaty with Belgium?

They did, and it was correct. (In France, the argument was even simpler: Germans were invading the country.) What they did not know is how these conditions had come about, and what political alternatives there had been.

German soldiers had been told that they were preventively acting against domination by Britain and France, plus a pan-Slavic conspiracy, and given similarly selective information (by persons of authority who should have known better).

In other words: I would not discuss about soldiers in that context at all. Discuss governments, newspapers, churches, and other forces of society. Take the Iraq war as an example for some aspects - we still remember it.