英语中，could have done （包括 could have been），有以下两层含义。

1. 对过去情况的可能性推测，意为“（过去）可能做了某事”：

The room was so still that you could have heard a pin drop.

那房间静得针掉在地上都能听到。

Few could have foreseen the track of his career at the start.

很少有人在一开始就能预见到他的职业生涯的轨迹。

He could have hugged himself at his success. 

他很可能因他的成功而沾沾自喜。

Nothing could have been more annoying than the party last night.

没有什么能比昨晚的派对更烦人了。

【注】你的问题就属于这种情况，句中的it 指“攻击的目标”。

2. 表示与过去事实不符或相反的虚拟语气，意为“（过去）本来能够做某事，而实际未做到”：

I suppose I could have loved her.

我想我本来可以爱她的。 （实际情况是，过去我并没有爱她）

She could have done well if she had got our help.

如果得到我们帮助的话，她就能够做好了。（实际情况是，她当时没有做好）

The show could have been much better. 

演出原本可以更好的。（实际情况是，演出结果没有更好）

The accident could have been avoided, but he was too careless.

这个事故本来是可以避免的，但是他太粗心了。（实际情况是，事故发生了）

【注意】有时候，一个句子上述两种意思兼而有之：说推测也可，说虚拟也行。这需要根据上下文来判断。例如：

He saved her life when she could have been drowned.

【翻译1】在她可能被淹死的时候，他救了她的命。

【翻译2】他救了她的命，当时她本来会淹死的。