@dorster
That is simultaneously fascinating and totally not what the OP is talking about.
It is however an interesting thought experiment. Of the 505 stocks in the SP 500, how many of those companies would continue to operate their overseas holdings as publicly traded companies if the US nationalized the US holdings? Would anyone honor my stock?
Typically, even in banana republics, when a company gets nationalized there is at least a token payment to the shareholders. In your thought experiment, I suspect your stock would be worth whatever the ex-US holdings are worth, minus the token payment.
However, I think we are getting a little off into the weeds. In theory, the US stock market could go to zero, but the reason will be something so crazy none of us have thought of it. The only way to mitigate against something like that is build a bunker in the back yard and even that might not be enough.
If you own the broad market, you can be reasonably assured all those companies will be trying their damnedest to make about 7% profit every year. Actually, they will be trying to make more that that, but on average over time that is what they will actually make. If history is a good guide, of course. Stock prices can be whacky in both directions, either way too high or way too low, at any given time. And they can stay whacky for long periods of time. But over any reasonable length of time, stock returns fall into line with corporate profits, which are about 7% a year. We don't have control of the wackiness, but we do have control over the holding period, and the longer the holding period the safer it gets. That means the best time to buy stocks is today. Next best time is tomorrow.
I say this as a person who has been through not only the Great Recession and housing bubble, but the dot.com bubble, the Asian Contagion, the Russian currency crisis, and few other panics that almost everybody has forgotten about today. The short term shit like we're seeing this year is absolutely, completely meaningless to your long term goals.