force majeure
(French law 法, 律) unforeseen circumstances, such as war, that excuses sb from keeping a promise, fulfilling a bargain, etc 不可抗力(难以预见的情况, 如战争, 藉以原谅某人未能实践诺言、 履行协议等).
On our way home that night I looked down and saw that the necklace was not around my neck any more. I told Pierre. We rushed back to the palace and looked for it.We asked everyone there if they had found a necklace, but without luck. We couldn't find it;