I believe they will only come at us obliquely as we are seriously tied up elsewhere
That happened once before.
Ask the Japanese how they liked the results.
Well, China is satisfied with the result of Korean War.
But they would pose a serious threat. We lost a lot of good people, and were seriously threatened while putting the Nazi and Imperial genies out of their misery.
I believe such an occurance would pose an even greater threat today, but would eventually have the same outcome. That's what the book series is all about.