You've heard the axiom "History repeats itself." It does, but never in exactly the same way. To apply the lessons of the past, we must understand the differences of the present.
During the American Revolution, the British came prepared to fight a successful war—but against a European army. Their formations, which gave them devastating firepower, and their red coats, which emphasized their numbers, proved the exact opposite of the tactics needed to fight a guerrilla war.
Before World War I, generals still saw the cavalry as the flower of their armies. Of course, the horse soldiers proved worse than useless in the trenches.
Before World War II, in anticipation of a German attack, the French built the "impenetrable" Maginot Line. History repeated itself and the attack came, but not in the way they expected. Their preparations were useless because the Germans didn't attempt to penetrate it; they simply went around it, and France was defeated.
The generals don't prepare for the last war out of perversity or stupidity, but rather because past experience is all they have to go by. Most of them simply don't know how to interpret that experience. They are correct in preparing for another war but wrong in relying upon what worked in the last one.
Investors, unfortunately, seem to make the same mistakes in marshaling their resources as do the generals. If the last 30 years have been prosperous, they base their actions on more prosperity. Talk of a depression isn't real to them because things are, in fact, so different from the 1930s. To most people, a depression means '30s-style conditions, and since they don't see that, they can't imagine a depression. That's because they know what the last depression was like, but they don't know what one is. It's hard to visualize something you don't understand.
Some of them who are a bit more clever might see an end to prosperity and the start of a depression but—although they're going to be a lot better off than most—they're probably looking for this depression to be like the last one.
Although nobody can predict with absolute certainty what this depression will be like, you can be fairly well-assured it won't be an instant replay of the last one. But just because things will be different doesn't mean you have to be taken by surprise.
To define the likely differences between this depression and the last one, it's helpful to compare the situation today to that in the early 1930s. The results aren't very reassuring.
Banks, insurance companies, and big corporations went under on a major scale. Institutions suffered the consequences of past mistakes, and there was no financial safety net to catch them as they fell. Mistakes were liquidated and only the prepared and efficient survived.
The world's financial institutions are in even worse shape than the last time, but now business ethics have changed and everyone expects the government to "step in." Laws are already in place that not only allow but require government intervention in many instances. This time, mistakes will be compounded, and the strong, productive, and efficient will be forced to subsidize the weak, unproductive, and inefficient. It's ironic that businesses were bankrupted in the last depression because the prices of their products fell too low; this time, it'll be because they went too high.