Use of "then'" in the sentence

In these dispositions then has a different reference and significance.

Set off from the sentence with commas, as in your first sentence, then is still a sentential adjunct but it has the meaning “thus, consequently”, drawing a conclusion from the matter which has gone before. Your first sentence may be paraphrased:

So the emasculated society of Europe serves as a warning to conservatives ...

Then preceding an attributive adjective, as in your second sentence, modifies the adjective with the sense “at that time”. Your second sentence may be paraphrased:

The society of Europe, [which was] emasculated at that time, serves as a warning to conservatives ...

In your second sentence the comma after Europe should be deleted.

The then emasculated society of Europe serves as a warning ...