Most of the world's banks, whether big or small, are located in the US and Europe, In case of war, all banks will be closed, Which will immediately benefit Bitcoin, The price of bitcoin will increase infinitely, Many banks will quickly print dollars and euros to buy cryptocurrency quickly, This will have a huge impact on Bitcoin as there will be no other source for cross-border transactions. Remember that most of the world's banks are located in the US and Europe, while 20-25% of them operate in Asia and other regions.
If 1 BTC is worth $1 Billion then the number of billionaires on earth will increase just like the number of millionaires are increasing now. And the upper people sitting in finance don't want it because they don't want lower class people to go up using this technology even if many already reached the step of $1 million dollar using this technology but the gap between $1 million to $1 billion is not small.
There is no way that BTC could reach that point because to touch that point millions, billions, trillions of money should be added in BTC's market cap. As of today market cap is around $815 billion and in my opinion, the market cap should be around $20 trillion for BTC to touch the $1 billion target per coin. And gaining a $20 trillion market cap is not an easy thing.
In the war other than banks many other sectors of life will also be destroyed and not to mention that most of the nodes are in these regions and if the electricity is gone due to war and no internet will be provided or if mining machines will be destroyed in the war, then the network will become more congested and speed will be slower than before.