。
The spirit of the treaty was being forgotten.
Literal
Treaty [genitive-の] spirit [subject-が] obliterated-passive [progressive-ていた].
Formal political/legal writing. 没却する ('to forget completely, disregard, obliterate,' literally 'to sink into the void') is a rare formal verb, more literary than 忘れる. In passive form 没却されていた ('was being forgotten'), it gives an indictment-like tone — common in political criticism about broken agreements or abandoned principles.