Why? Currently, Japan imports literally most of it's food. (See for instance this newspaper article, which begins,
The government’s new target of raising Japan’s self-sufficiency in food to 45 percent in 10 years — compared with the previous target of 50 percent — may be more plausible because it’s closer to reality.
Furthermore, as a nation I believe they're pretty conscious of this fact (as the existence of the above snippet hints) so it makes perfect sense to me that it would be at the top of the national agenda in this instance.
And it's a pretty legitimate concern. Going to half-rations is already pretty serious business—Japan doesn't overeat much in the first place, I think—but agricultural yields could drop significantly if the modern farming equipment starts to fail and stocks of certain agricultural supplies go down. It's not as simple as just planting more fields; not when you've got 120 million people to feed.
(And yeah, that brings some focus back to figuring out the machinery and industrial supply problems, but opting to grab some stuff from your medieval neighbors instead seems reasonable. What isn't reasonable is... well, I already went into that.)
@Pokari All true but in terms of arable land and having an agricultural society advanced enough modern farming practices, they are capable of becoming self-sufficient with no land acquisition. It's not like China or India where farmers are still stuck in the feudal age and a single drought will cripple them.
So, along with Kuril islands russian military was teleported as well, right? Or they were dropped off in the middle of ocean since russians are not isekai-worthy?
I hate reading stories like this, not because of the content, but because of who is writing it. It screams of Japanese atrocities that they refuse never happened but the rest of the world knows did.