how to understand this sentence?

which代的是A disaster。A disaster happens either through culpable lack of foresight or through adverse external agency and brings with it destruction
either……or……意思是不是……就是……
天灾的发生不是由于由该受责备的缺乏远见就是由于不利的外部影响所带来的破坏

下面是大家网英语专家的解答：
A disaster is an unforeseen mischance or misadventure (as a shipwreck, a serious railroad accident, or the failure of a great enterprise) [which happens either through culpable lack of foresight or through adverse external agency and brings with it destruction (as of life and property) or ruin (as of projects, careers or great hopes)].

which 引导的是定语从句，修饰前面的mischance or misadventure，全句意思是：

灾难就是不可预见的不幸或遭遇（如轮船失事，重大铁路事故，或大企业的失败），其因不周到的缺乏预见，或不利的外部因素而导致发生，并随之带来摧毁（如生命或财产）或灭失（如工程，职业，或希望）。

全句主干成份是：
A disaster is an unforeseen mischance or misadventure...

which从句结构已经清楚标明，呵呵！

后面的：
brings with it destruction (as of life and property) or ruin (as of projects, careers or great hopes)
正常语序应该