'It's in the middle of our capital city, go away.'
"Due to violation of treaty commitments with the United States and allegations of war crimes that the Japanese government has thus far been unable to adequately prove false due to not allowing international observers, the UN Security council has passed a resolution enacting trade sanctions of militarily important goods to Japan. The expected effects on Japanese industry are..."
Alternatively
"Would you like it not be
your capital anymore?"
Both of those are of course unrealistically far beyond the point where Japan would go "Yeah, actually, you can send some observers over to keep an eye on things but its still
our playground" and the rest of the world goes "Yeah, yeah, we're cool with that. Hey, our corporations have special skills for setting up the infrastructure you need over there that would be
super useful for you *wink**wink**nudge**nudge*"
Japan not letting
anyone in is frankly stupid. One, the attack on them would have their allies lining up to help both for selfless and selfish reasons and them keeping the US out is basically impossible. Two, the implications of the gate are enough that the superpowers would want their piece of the action and would absolutely be willing to stick the global economy on the line to get Japan to back down and let them in. Again, you can also have the juxtaposition of the Japanese rankling over the fact that you have foreigners setting shop in their capital because they aren't strong enough to resist them showing up with the fact that they are setting up shop in Alnus because no one there has the strength to resit them to say something about the allure of hegemonic power and how it is also not nice to have it applied to you.
If the series were smaller, like say a significantly stronger force of monsters emerging through the gate and the focus was on the JSDF fighting back then and there then there would be more to be said about keeping it focused solely there, but the series lets the scope expand beyond that. That it feels the need to address the elephant in the room of "What's the rest of the world doing with regards to this world shaking development?" by taking the least realistic approach that only works to make Japan look strong and the rest of the world look foolish is one of the big things that weighs the series down. It could go with "Japanese politicians are skillfully playing the US, China, and Russia off each other to keep them more interested in interfering with each other than with us" which would be
great since it is at least a realistic sort of Japanese chest beating... but it has to go down the tired right-wing "Politicians are stupid and interfere with military operations and so have to be kept out of things and put in their place" stance so having the politicians skillfully politic doesn't really fit into things.
Like, even Stargate had the Americans in the story bringing in the Russians and Chinese eventually because the implications to the rest of the world were too much for them to keep the rest of the countries out forever without risking a war they couldn't afford. The other countries didn't really come up much until Atlantis where it was a fully international expedition, but the show at least acknowledged that they had an interest in the situation. Hell, you can keep it as low as "There's a bunch of UN observers at base camp and we've been given licenses for US military hardware. Seems they're willing to sell us weapons but don't want to put their
own people on the line. Still, gonna be a whole lot of new factory jobs, going to make the voters
real happy on that. Keep the observers happy but keep them from getting their noses in things." Boom, international implications addressed, don't have to add anyone non-Japanese in unless you want to have political shenanigans, and you can remind people of Japan's strong industrial advantages while also subtly putting down the rest of the world for not wanting to get their hands dirty directly. You can even do things like having a CIA op fuck up during negotiations with the Empire add unnecessary complications and be able to have Japan chide America in a way that the rest of the world goes "Yeah, can totally see that happening, fucking Yanks" instead of the whole "Only America uses black soldiers!" thing.
I think the big thing is that its so much more interesting when people are being intelligent but don't have all the information so their fuckups are realistic and there can be some narrative heft to things. People see all of this
potential and how it is being squandered on lame "Japan STRONK" shit. It's the frustration of "You're
good but you can be
better!"