https://www.youtube.com/watch?v=W8a7gpoSPbY
the other has details about how it was made and a link to a paper about the steel:
The main thing I've heard is that in Europe all the best swords used spring steel, which is hard enough to cut really well while also being able to flex without getting chipped, broken or bent. Japanese smiths couldn't make spring steel for various reasons, so as a workaround they used differential hardening, etc. to make the cutting edge super hard, while the back & sides ended up more ductile. It works pretty well, but the cutting face is still a little bit brittle & the sword as a whole ends up heavier & vulnerable to getting bent b/c it doesn't snap back into shape like spring steel would
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Unlike what some say there, the fascination of Westerners for Japanese swords has not started after WWII. Already during the 19th century, the first Europeans and Americans who could visit Japan were all extremely impressed with the Japanese swords, as demonstrated in test cuts but also by cutting parts of human bodies in some incidents.
Even if in the more distant past there have been many places where swords of very high quality had been made, e.g. the Toledo swords or the Damascus swords, by the time of the 19th century such techniques had been forgotten for a long time and the remaining production of swords was oriented to the mass production of cheap but low quality swords. Such swords were more resistant to mishandling than the Japanese swords, but they could not compete in hardness and sharpness.
The same travellers who were impressed in Japan by the Japanese swords were not impressed at all by the swords encountered in the other countries of the region, e.g. by the Chinese swords.
At least by the time of the 19th century, in no other place were there smiths willing to spend so much time with the sharpening and polishing of extra hard steel.
Even today, when everybody knows modern metallurgy, using Japanese kitchen knives is a very different experience from using European kitchen knives (at least for me, one much more pleasant), due to the Japanese preference for maximum hardness and sharpness, despite the fact that such blades require much more care during use and maintenance.
Because the rest of the world used rifles as a battlefield weapon, maybe.