I think none whatsoever, most people will not even notice and those who do, will forget five minutes later after the next reality show starts. The US was routed in Vietnam and what happened? Nothing for a while, and then we had the 'Reagan revolution.'
While you mentioned WWI - what propelled the development of fascism in Germany was not being routed in WWI but the Great Depression or rather the combination of the two. However, Great Depression the US had the opposite effect - it brought the New Deal - perhaps because the ruling class was unprepared (albeit their tried to organize a coup, but failed).
In sum, unless there is a major recession cum international setbacks in the pipes, I am not going to loose much sleep over the possibility of a martial law in the US of A.
Wojtek