If i remember correctly, in our world history, in ww2, when Germany have conquer France....they somehow let some of France soldier and government escape and later those who escaped declare to be government in exiles and continue to fight with Germany. And much later be able to combine force with the allies and beat Germany back.
I think Tanya fear for that very scenario.
It's not somehow, but the result of the over confidence from field commanders and the ignorant of the top brass.
The field commanders did even worst, they expected swift victory and pushed too far, putting great strains on logistics. This is why they had to stop at the outskirt of Dunkirk, HItler's commands only served as the final nails on that coffin.
Then, the top brass expected that even the United Kingdom would not want to mess with a power strong enough to take out France, their bitter enemy for hundreds of years in a mere 6 weeks. For that, they did not think France would keep up the resistance in exile, like the Polish did.
If anything, the very fact people often over looked is that Germany lost the moment they let Polish government run off in exile. That created a precedence that was followed later on and brought even more powers into the fray after that.
It's not "somehow", but because of a lot of negligence.
It's EXACTLY what our protagonist is afraid of.