To add to what others have pointed out, there’s also the fact that American unity is precarious. Most young Americans don’t really feel much sympathy for the institutional framework of the US, don’t have any incentive to maintain it, and lack any kind of unified social or cultural framework to form an attachment with. The problem with implementing something like Totalförsvar in the US is the fact that most Americans don’t actually like or want to protect their fellow Americans in this era. Why would young people want to participate in a civil or military defence schema to defend a paper-country that doesn’t benefit them in any coherent way? People absolutely will serve to protect their own clan or tribe, but the idea of sacrificing for the sake of “Those People” (for whatever flavour of “Those People” that individual resents the most) would create such a spectacular backlash any such proposal would be dead on arrival without some kind of massive financial incentive.

National Service schemes only work in the context of effective social welfare systems, and there’s nothing the US hates more than the idea of effective social welfare systems. Desperation is an essential part of their economy, as very few of their current crop of corporate “leaders” would be able to employ anyone at all if the alternative weren’t starving in the street.