One potentially important difference between 1918 and now is that in 1918 quickly traveling over long distances was impossible and,for that matter,international travel was almost unheard of. Today,1000 people a day (or more) can travel from,say,Bangkok to the US in less than 24 hours.Those people can all be infected but asymptomatic both when they leave Thailand *and* when they arrive in the US.
Your point is well made. So why have we not shutdown international flights if it's so inevitable?