一段文句
The criteria – a route to send defensive supplies to northern military bases
should mainland US or Canada be attacked, far enough inland that enemy planes
couldn't reach it – made the proposed route starting from Dawson Creek the
top contender.
出處
https://reurl.cc/NpxrAp
請教這邊should mainland US or Canada be attacked該怎麼解釋?
是修飾military bases嗎?
相關的文法應用該怎麼參考?

should這裡跟if差不多意思。查字典應該可以查到這種should的用法。

if mainland US or Canada should be attacked倒裝句，指「如果……發生的話」

should had 等助動詞假設語氣可以倒裝這種用法會話也會用到喔 建議學起來

had, should, were

謝謝各位