What is the meaning of " such as" in this text?

Question

No difficulties had delayed the invaders but such as Nature had interposed to thwart them.

Answer 1

"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.

Answer 2

Such is a fused-head construction in this sentence (the understood head of the noun phrase being difficulties). The prepositional phrase headed by as contains a comparative clause and modifies such.

such difficulties -> difficulties as Nature had interposed (difficulties)

No difficulties had delayed the invaders but such (difficulties) as Nature had interposed to thwart them.

This construction is very similar to a relative :

...but those which Nature had interposed to thwart them

There is no difference between the interpretation of the original and that with the relative constructions substituted. This use of such as is rare, definitely formal and probably bordering on archaic.

Then, going a few steps forward, he would pause at a heap, and begin counting cinders into his basket, as though it had been so much solid coin . Such as knew the broker heaved a sigh of compassion. (The Career of Puffer Hopkins; Mathews, Cornelius; 1842)

relative: Those who

No refuse or worked objects of any kind were found in this black earth, except in the recess in the east wall, as described, and in the upper portion immediately under the ashes . Such as existed outside the recess may have become mixed in the same way; that is, by being thrown on the top as it existed at the moment and being later covered by the water; or it may have worked in from the ashes above. (Archeological Investigations, Bureau of American Ethnology, Bulletin 76, 1922)

relative: Those which