The clue to the meaning is in the second sentence, "It was the Germans, the Germans." The way the character speaks, as well as the mention of failure, makes this sound like the Germans are to blame for something.
If the mission itself were poor, meaning it lacked resources, then the blame would fall on the mission's leaders. If the leaders had failed to equip the mission properly, we would expect them to be named characters, and not simply "The Germans".
If the mission were on behalf of the poor, a native speaker would say "the relief mission", "the charity mission", or more likely just "the mission". Also notice that the author is focused on the failure, not the details of the mission.
I believe that the author is using a third definition for "poor", meaning unfortunate or deserving of pity. This echos the line about failure, and the character's death, as well as explains why they might blame "the Germans". In this interpretation, we might rephrase the sentence like so:
"The mission came to a tragic end," he said, and - "It was [due to the actions of] the Germans."