They cannot. This is unrelated to Bitcoin, as Bitcoin isn't "American". Even various countries banded together to attack Bitcoin, they would not succeed. Annoy yes, but not destroy Bitcoin. It was designed to resist such a thing since the beginning, when it was though all the govs and banks would go against it (they didn't).

Funny thing is, if China went to war against US, they wouldn't attack Bitcoin. They use it too you know? I don't see what makes you thing Bitcoin is somehow American, when many American politicians and bankers and rich people hate it...

Attacking Bitcoin is costly, and the more you try, the more you spend. It would bankrupt your economy before you could achieve anything. So forget it.