This is a great chart from Patrick O’Shaughnessy on worst case returns for stocks. As you can see, the US had pretty good returns for the past 112 years, but even then there was a 20 year period where you still lost money. In fact, MOST countries had a 30 year period where you could have lost money- ouch!
PS you may as well go ahead and pre-order his book, out in October. I’ve read it, and it’s great!