What is the meaning of " such as" in this text?
No difficulties had delayed the invaders but such as Nature had interposed to thwart them.
"Such" refers to a type of something previously mentioned, for example:
-Have you experienced any difficulties of nature?
-I have experienecd no such difficulties.
"But" introduces an exception or a contrasting clause to what has already been mentioned. So, your sentence essentially means that no difficulties had delayed the invaders except the kind caused by nature.