To look at this in another way, you may say that the death of nearly anyone born after the start of WW1 was caused by WW1.
Which child is actually conceived is extremely variable, being dependant on a huge number of external factors such as who the parents are, timing of conception, numerous environmental factors, personality and mood of parents, etc.
An egg being fertilised by one particular sperm that would make one particular person depends on coitus happening at a precise moment by particular parents exposed to precise environmental and historical circumstances. Even the make-up of the particular egg and sperm will be influenced by infinite past factors. Then also the development of the foetus will be influenced by numerous further factors.
It is easy to imagine that in some way every potential parent after 1914 was in some way influenced by the Great War; a change in their life changes the circumstances and timing of the conception of their offspring. Perhaps some people living in the most remote places on Earth may have had no initial influence on their existence by the happenings in Europe but since then the effects would accumulate.
So, if the birth of a particular individual is dependent on exact circumstances then alterations to any of their circumstances would very likely result in a different individual being born. A huge event like the Great War would affect most humans on the planet, leading to this event causing entirely different humans to exist than would have if the war had not happened.
My limited understanding of Chaos Theory would suggest that any event could trigger a cascade of influence that could change which egg and sperm come together. For example, a woman in Croydon choosing to wear her red dress one day rather than her blue one might increase her attractiveness to a particular man, leading to a relationship, marriage and children. This effect rippling out across Croydon to England and on out to all humanity. I imagine this effect being relatively limited though; however a world-wide event like WW1 would have a huge effect on humanity.
If all or most existing humans now only exist because of WW1 they could then only die because of the war. So, anyone at all who dies at this instant is doing so only because of WW1.