>If there's a world war and all of a sudden Japan doesn't have computers?

Pretty much. If you're at war you need that materials to feed the war machine, which includes the metal to build the plane as well as the computers that go in the cockpit (and let's not forget the knowledge to build it all).

Got it.

Kind of makes sense.

I don't think the issue is actually national security as much as preserving Japan's relatively closed economic system of interlocking corporate ownership / keiretsu.

Why am I being down-voted? That's the standard bargain Japan has with the US. It goes back to the Cold War. I'm not passing judgment; it's just a known matter of fact that their economy is rather closed to outside investment, especially controlling interests.

They just don't want the Chinese to get DRAM fab technology and then flood the market with cheap parts.

Perhaps they meant economic security.