It depends on your objective unless it is to maximize the average estate you leave for your heirs, I think 100% stocks is a really bad idea. When you are in the accumulation phase 100% stocks is just fine, I think it probably is optimal AA for hitting critical mass the quickest. A heavy stock concentration say 80-90% is also fine if you are an early retiree planning on 3.5-4% withdrawal rate with much longer than 30 years retirement, e.g. Mr Money when he first retired.
However, if you only need to withdraw 2.5% then you definitely want to diversify across both countries and asset classes. It's worth noting that 4% SWR rule applies only to the US, and some Commonwealth (Canada, Australia, maybe New Zealand), a minority of European countries, and very small number of countries in the rest of the world.
There is probably no SWR rates for 1930s retiree in Germany, Poland, Italy, Romania, Hungary, Bulgaria, Japan, or China with 100% of their money in their countries stock market. I imagine the SWR for a French, Belgium, stock investor is very low. And it is not just war. a Venezuelan retiree in 2000 would almost certainly be broke by now, even though the Venezuelan stock market was #1 in the world in 2017, up over 3800%, but with inflation, at 7,000% they lost a ton of money.
The Dow lost 90% during the Depression, the market rebounded pretty quickly which is what would have saved the 1928 retiree. There is no guarantee that market doesn't take a few years to rebound the next financial crisis. Bond did relatively welll during the Great Depression.
Bonds certainly can one of the other asset classes you diversify into. But, I'd argue if you really want to protect yourself I seriously also consider things like real estate, direct investments in business which is what I've done, along with things like gold, Oil, or maybe even crytpocurrency,