There are really two categories to "war resistance". Did this war completely knock out the internet? Then your limited to satalite, meshtastic, physical radio tech.Maybe jack dorseys bitchat would work since everyone will already have the hardware to build the network out.
If you have internet...then Tor, I2P, and other overlay networks will remain very resilant and allow you to communicate over any distance...that's what it was made for.
Severing fiber connections between Sweden and central Europe won't matter to Tor. It's whole job is to make secret unblockable tunnels.