REST - What should DELETE /collection do if some items can't be deleted?

I have a collection of items and some of them may or may not be deleted, depending on some preconditions. If a user wants to delete a resource (DELETE /collection/1) and there are external dependencies on that resource, the server will return an error. But what should happen if the user wants to delete the entire collection (DELETE /collection)?

Should all the resources which can be deleted be deleted and the server return a 2xx, or should the server leave everything intact and return a 4xx? Which would be the expected behavior?