So according to the title you want people start panicking about their kids boxing and fencing and wresting?
Martial Arts consist of more then just the Eastern versions, the West has some pretty good art forms as well.
I guess the POD is the Central Powers winning both world wars, the second against a vengeful Britain. Is Peru and India socialist? what is that Ukraine-Caucasus state? what is the situation in East Asia?
How exactly is the rest of the world worse off than Europe? The Nazi empire and Japanese clearly had tech regression because they are idiots. but why did The United States get WORSE?
I think the hundred years war thing works. An on-and-off conflict between the two sides: America, UK and united West Europe vs the Kaiserreich, Turkish empire, and China, the rest of the world is a warzone
I think a major change would actually be its effect on high fantasy. In the Hobbit and Lord of the Rings the Great Eagles are considered an analogue of Americans. If the national animal was a snake, perhaps the eagles would be replaced with a race of friendly dragons that oppose Smaug and...