谁给分析一下这句英文倒装句的结构

问题描述:

谁给分析一下这句英文倒装句的结构
because at no previous time has American security been as seriously threatened from without as it is today.
来自 Roosevelt 1941年的国会演讲

at no previous time 属于否定词提到句首,采取部分倒装,正常语序为:because American security has been as seriously threatened from without at no previous time as it is today.