In any significant war the Internet is going to go down. That's what has happened empirically in countries undergoing significant wars or social unrest, like Russia, Iran, Yemen, Ethiopia, Syria, Myanmar, and Afghanistan. While IP packet routing itself may have been designed to survive a nuclear war, there have been many centralized systems built on top of it (DNS? Edge caching? Cloudflare? Big Tech) that are essential to the functioning of what we know of as the Internet.

If your threat model includes war and you want to have some of the conveniences of the Internet, you should make plans for how to host local copies of data and develop local-scale communications for the people you regularly talk with. The Internet is too big of a security and propaganda risk for governments to allow it to continue to exist when they are engaged in a real existential war.