If you haven't watched the anime, you should, it's good, but, to sum-up: a "gate to another world" opens in a shoping district in Japan, and an invasion force pours out of it... With Spears, and axes, and stuff. They kill some people but are slaughtered within hours by the JSDF on account of "You're a dude riding a dragon, I'm an F-16 pilot, how did you THINK this was going to end...?". The JSDF, for intuitively obvious reasons, imediately counter-attack through the portal into the other world after moping up the invasion to secure the other side. (#Pilot-Episode spoilers.)

That's just the basis of this hypothetical, though: what if that, but not one portal, but THOUSANDS, across the entire earth, each of them leading to a diffrent paralel reality via a series of "dungeon like" caves and subteranian chambers full of monsters and such that impeed progress from both sides: and the worlds they conect to are ones from longstanding established fiction generes. As an example, one West Verginia might lead to a world that is ACTUALY "Fallout 76", while one in the Closet of Tolkiens old house in England might exit into Bilbo Baggins old house at "Bag-End" in "The Shire"; and such-like.

All the magic and crazy tech works in the other worlds, but can only be MADE in the "right" world for that sort of thing; although various "alternate versions" loosely based on it are likely to be possible with diffrent world's systems and tech/magic.

Only VERY popular and well-known fantasy/sci-fi planets/galaxies get a portal, or at least ones that once were, (The world of "The Heralds of Valdemar", for example); and, if the Earth canonicaly exists in that universe that's where the other end of the portal goes. (As an example, Nurin of "Skyrim" fame will have a portal, and so would "Fallout", likely more than one each, because in spite of rumors to the contrary "Nurin" and "The Wasteland" aren't the same universe in this scenario even though they are by the same company and such: but, in "Star Wars", Earth Canonicaly exists in the "Unknown Reaches" area of the galaxy, so the portal won't open on Corescant or Tatoine, and WH40K the portals would open onto "Holy Terra" since it's canonicaly our own future in that universe...)

Under these cercumstances:

A: what happens next, in your opinion, on the national/international level?

B: What would you do if this ocurs tomorrow with a portal opening in the closet of your bedroom? (Or equivalent.)