The ‘arrangement between Germany and Russia’ referred to here is the German-Soviet Frontier Treaty, signed on the 28th of September. Although Thomas states here that the terms of this agreement have been released, this and the preceding Molotov-Ribbonbtrop pact had a number of secret clauses, the existence of which did not become apparent until after Germany’s surrender in 1945. These protocols concerned German and Soviet spheres of influence and the division of Poland after the invasion.
Thomas expresses his shock and sadness at the fall of Warsaw. Warsaw was devastated by the siege and fighting in its streets. Around 18,000 civilians were killed during the siege. 10% of the city’s buildings were utterly destroyed with a further 40% heavily damaged. It is clear from Thomas’ response that even a veteran of the First World War found this level of carnage shocking and this feeling is very likely to have been widespread.