Perhaps the simplest way to go about understanding this, is to ignore individual spokes for the moment.

Where the wheel touches the ground, there is a force on the rim inwards. As you might expect, in that spot there is a small amount of inwards deformation of the rim (think very small). This deformation is local only, since the force is local. So, at this local area of inwards deformation, between the rim and axle, the wheel structure is experiencing a small compression - the distance between axle and rim becomes less.

Now think for a moment how wheels are made. Initially, wheels were solid discs and that presented no problem to understand. If you overloaded your wooden disc wheel, the axle would crush through the bottom bit of the wheel and you would curse and go and cut a new disc.

Some bright spark thought that wheels might work equally well if the were made from wooden spokes instead of a disc. It was just a problem how to keep those wooden spokes in place, so they came up with the idea of a steel tyre which is slightly too small, so that it puts the wheel structure under radial compression, keeping everything together.

Still later the idea surfaced of making a metal spoked wheel. The new problem was, if you make the spokes thin enough so the weight is acceptable, they would simply buckle under load. So the next bright spark thought, well what if we put those thin metal spokes under pre-tension, enough to cancel out the load force that would buckle those spokes? Well as we all know, this was a great idea; a bicycle wheel is an enormously strong structure while being very light indeed, all because of thin wire spokes under tension.

The problem has not changed since the old days: If you overload a wheel it will crush through the bottom part of the wheel. The wheel pieces are not strong enough by themselves.

So what typically happens to a wire spoked wheel, is because the bottom spokes some lose tension under load due to the local inwards deformation, there is a maximum load where they lose all tension and they will immediately buckle if the load exceeds the maximum. When this happens, the wheel has lost all capability to resist further load and the rim at the bottom will deform inwards, in effect dropping the axle downwards (same as crushing your wooden disc wheel). Deform far enough and the rim suffers permanent deformation or under very severe conditions, it will completely collapse inwards at the bottom.

Spokes break due to metal fatigue at the elbow. With each wheel revolution, each spoke undergoes a small loss of tension as it passes through BDC; and each time the elbow bends to and fro a tiny bit. Do that enough times, and the tiny cracks from metal fatigue will break off at the elbow. So it makes sense to design spokes with no elbows. This also a very old idea - check out this window made form an old wheel - no elbows and nipples at the hub, all seemingly modern ideas:



So while wheel theory has not changed, the clever idea of pre-tensioning spokes has caused enormous confusion of what actually happens in a metal wire spoked wheel.