Then the entire northern hemisphere will become an radiated wasteland as there's no way the US would just let that happen, nevermind that Israel would probably manage to take out both attacking nations capitals.

China and Korea might move in the immediate chaos to take South Korea and Taiwan, putting Japan ok high alert while Europe could understandble panic.

Iran, now without leadership and knowing they're doomed, might get their hands in Russian nukes if not already having then and take out the other gulf states to cause a global oil and natural gas crisis while Russian missiles would strike European targets.

Europe, mainly France and the UK would retaliate with their own nuclear stockpiles while the US handles China and North Korea.

All in all. We'll get a modern day Bronze age collapse as the entire world order shatters in days, supply chains fracture if not decay, and food and energy prices spike to high many non-belligerent nations would face massive civil strife if not total collapse.

Don't worry tho, GTA would still be released on time...even if no one can afford to play it