Sentence:  The fact is incontestable : that liberal world of Unitarian Boston was narrow-minded , intellectually sterile , smug , afraid of the logical consequences of its own mild ventures into iconoclasm , and quite prepared to resort to hysterical repressions when its brittle foundations were <WSD>threatened</WSD> . 
Meaning of the word marked with <WSD> tags:  pose a threat to; present a danger to