Although none of us want a world war to happen, because war never brings peace. But yes, your idea is right, during the war, the value of the country's fiat currency will definitely decrease and can that currency disappear if the country is destroyed. Yes, but if you have a good amount of Bitcoin. Then if you save your life, you can definitely do something. But I am worried that if there is a world war, will the servers of big giant companies be destroyed?

I think ABCBitcoin's answer about brainwallets for refugees is the best one until now.

I don't think Bitcoin will stop working and probably not even the hashrate would suffer that much. The exception would be a situation like pre-2021 when really 80% of mining were concentrated in a single country, and of course if just this country is under attack then the mining facilities could become targets. It would however perhaps be more lucrative to try to gain access to the miners' wallets via cyberattacks or intelligence work than simply destroy your adversaries' mining farms.

While I don't think that banks will be destroyed by a world war, what could happen is indeed a hyperinflation in one or several of the participating countries, leading to a currency reform like in Germany in 1948. If such a situation can be predicted, it's even possible that Bitcoin's price could benefit, as would gold's price.

International commerce in times of currency volatility due to economic warfare is another use case. And of course if your country is occupied, you could use Bitcoin (or Monero) to support movements and groups fighting for freedom, when using traditional banking has become too dangerous because the occupying country strictly monitors all local banks.

Of course this post considers that a "WW3" would be comparable in scale with WW1 and WW2. In both wars there were severe destructions but 80-90% of the world's population were almost unaffected by the warfare itself. WW2 for example was devastating in some parts of Europe and Asia but only punctually there was warfare in other areas, e.g. in Abyssinia, a small part of Northern Africa and Hawaii, but the US and India for example were barely affected. There are of course more apocalyptic scenarios where Bitcoin could take damage. However even in the case of a single 51% attack this wouldn't mean the end of Bitcoin; if it's only possible during an extreme situation then it may be possible to simply start the chain after the war (or when the situation improves) from a pre-attack block height. Even reverting the attack isn't impossible, Ethereum did that already once and they're still alive

First of all, I want to say one thing, that is directly or indirectly, World War III (WW3) has already started.
And many countries have already participated in it.
Russia has expressed support for Iran
China has already provided military assistance.and the United States has directly engaged in the war.
Although the two sides agreed to a ceasefire for a while, it didn't last long the missile attacks resumed again.
Now the question is  how can Bitcoin benefit everyone at this time?
First, the economic and moral condition of any country is severely damaged during and after war.
Even countries that have business relations with those countries are also affected. What are the advantages of Bitcoin in this situation?
I am presenting my own opinion .If the banking system collapses during a war, then it is possible to continue transactions very easily through Bitcoin.
After a war, a country has to print more paper money to restore itself, which is a major cause of inflation.
But Bitcoin has no chance of inflation, as its supply is limited (21 million).
Therefore, Bitcoin has many advantages during wartime and even in the post war period.
If countries like Iran or Israel have Bitcoin in reserve. then they will be able to quickly restore the country's economic situation after the war.
Even if their banking system is completely destroyed,
they will be able to continue to transact and trade with any country in the world via Bitcoin,
if Bitcoin is also legal in that country.

In any situation , be it war or crisis  Bitcoin has the power to positively change not only the economic situation of an individual, but also of a country.

This is not a world war yet.
A world war is not 5 countries fighting. It's much more than that.
During the WW2 the whole world was involved. There were a few neutral exceptions like Spain and Portugal, but you just cannot compare this to the current situation.
How many countries are formally in a state of war right now? The list is very short.

The main advantage of bitcoin is the way it can be moved and hidden. Moving wealth from one place to another once you run away from the front line is very hard.

Bitcoin is handy if wars happen the same way it is handy if they don't.  Governments print more money and Bitcoin goes up.  When wars happen, governments have to print a ton of money and then they have higher debt servicing loads that require even more money to be printed going forward.  Basically, the dumber governments act, the more bullish the case for Bitcoin.  Anyone who has been paying attention these last 24 years can tell you that government has gotten so stupid, there is literally nothing that can save it at this point.  Bitcoin is inevitable. 

The war and other things drag Bitcoin down to $98,000 before it now rises up to above $100k, which it is right now. War always has an effect on the price of things, which is not just Bitcoin, but I wonder why an asset like Bitcoin will always react to war like this when it could have been where some people could have been running to Bitcoin for saving their small money since the result of war is unknown and unpredictable.

If World War 3 happens, then I think there will be a lot of investors dumping their digital assets including bitcoin just to buy gold. I think gold tends to be safer as a store of value during a war, but gold is certainly very inefficient to move around since it is a physical asset. Non-physical assets like bitcoin tend to be good to have, but the impact of World War 3 will certainly make its price drop drastically so gold might be an option. This is just my assumption about what might happen, but I never expect World War 3 to happen anytime soon.

War will disrupt many things including internet and electricity. We don't know exactly what will happen, but I can imagine that the impact of World War III will be really bad for the world economy. Stock markets will crash and purchasing power will decrease, so in that situation will bitcoin become a store of value?

Well, if there’s a war in your country, it might be an effective asset since you can use it to buy necessary things to survive. But of course, it’ll depend if the market will go crash massively because it’ll ruin BTC, since the market always affected by the war then we’re not sure if BTC will be a handy asset during war. That time when US bomb iran, the market dips and everyone’s panicking already so what more if there’s a huge war between different major countries. Ofc, internet plays big part too, if they aren’t there then the BTC’s utility can’t be used.

Bitcoin's decentralized structure can provide some advantages in global crises such as war, but it should not be forgotten that it also has limitations. Bitcoin is not dependent on any central authority, bank, government.

In a scenario where financial systems collapse due to war or banks become inoperable in some countries, it may be possible for the Bitcoin network to continue to operate as long as there is an internet connection, if there is no internet connection, there is no access. If a country's currency collapses due to war, Bitcoin can be used as an alternative to the local currency, quickly and practically.

In my opinion, although bitcoin has the characteristics of a safe haven asset like gold, such as scarcity and decentralization. But the problem is that investors still don't fully believe in it.

War creates instability, investors will only look for safe assets to protect their assets and unfortunately so far it is probably only gold that they trust the most, not bitcoin or stocks. Many people call bitcoin digital gold, but in reality, they still consider it a speculative asset, so they want to sell it during wartime.