For want of a nail the shoe was lost.
For want of a shoe the horse was lost.
For want of a horse the rider was lost.
For want of a rider the message was lost.
For want of a message the battle was lost.
For want of a battle the kingdom was lost.
And all for the want of a horseshoe nail

This medieval parable presents some intriguing insights into the folkloric view of the nature of causality.
To start with it's worth stressing its universal relevance. Whilst the outcomes of modern battles are not determined by the loss of a horse-riding messenger, he has modern equivalents in glitches in electronic communications, malfunctioning computer etc..
On the face of it, the parable presents a solid chain of causes and effects with an air of inevitability about it; a prime example of a straightforward deterministic process. However, the inevitability could be questioned by applying the following conventional criteria of the causality theory. They are referred to in the Wikipedia entry on causality as follows:
Necessary causes
If x is a necessary cause of y, then the presence of y necessarily implies the prior occurrence of x. The presence of x, however, does not imply that y will occur.[20]
Sufficient causes
If x is a sufficient cause of y, then the presence of x necessarily implies the subsequent occurrence of y. However, another cause z may alternatively cause y. Thus the presence of y does not imply the prior occurrence of x
Contributory causes
For some specific effect, in the singular case, a factor that is a contributory cause is one among several co-occurrent causes. It is implicit that all of them are contributory. For the specific effect, in general, there is no implication that a contributory cause is necessary, though it may be so. In general, a factor that is a contributory cause is not sufficient, because it is by definition accompanied by other causes, which would not count as causes if it were sufficient. For the specific effect, a factor that is on some occasions a contributory cause might on some occasions be sufficient, but on those other occasions it would not be merely cont

So examining the chain of causes step by step using the above criteria might yield the following results:
1, For the want of a nail a horseshoe was lost:
In the context of the tale, it is implied that since the horseshoe was lost it is the loss of the nail the necessary cause, However, the loss of the nail did not have to lead to the loss of the horse sho i,e if the horse remains stationary. The loss of the nail could be considered as a necessary cause for the loss of the horseshoe.

2, For the want of a horseshoe a rider: (meaning the horse’s inability to carry on)
The loss of the horseshoe is a sufficient cause for stopping the rider but not the only possible one. Many other causes could have stopped his mission; eg he could have been intercepted by the enemy, falling ill... We have here a sufficient cause which by definition does not exclude other causes

3 For the want of a rider, the message was lost.
The loss of the rider is a sufficient cause for the loss of the message which however could have been lost in other ways; eg left behind at the staging post, fallen off... Again we have here a sufficient cause

4. For the want of a message, a battle was lost:
Here we get into the realm of contributing causes. The battle could have been lost even had the message been delivered. The army could have been defeated by being outnumbered, under-armed, outclassed...or by any combination of such cause.

5, For the want of a battle a kingdom was lost:
A battle lost seems to me to be a sufficient cause for losing a kingdom which however by definition does not exclude other clauses; coup, revolution…

So, using the above-mentioned causality criteria the inevitability of the loss of a kingdom due to the loss of a horseshoe nail is far from established. It becomes more tenuous along the chain of events. That of course, does not exclude the possibility that the consequences of the loss a horseshoe nail could be as described. Nevertheless, I consider the mentioned casualty criteria somewhat unconvincing. In the cauldron of cause and effects, I cannot imagine the situation where some contributing causes do not play a part. It is interesting to reflect that the analysis of cause becomes more complex as the universe evolves. And conversely, going back in time the analysis becomes simpler because the number of causes decreases. There could have been only one cause at the time of creation (if any)

The ‘for the want of a horseshoe nail’ highlight colourfully the important point about the way events in the world are dominated by ‘triggers’, that is, small events that cause other events bigger events, thus unleashing in the process amount of energy of many levels of magnitude greater than that of the trigger, Consider for example the flapping butterfly wing in the Amazon or the pressing of the nuclear button...