There are two sides to 'violence'. On the one hand, things like boxing or other violent sports, e.g. cockfighting, sate a desire for violence, even vicariously, e.g. to be 'on the winning side' in a boxing match. If that desire isn't somehow given a vent in this way, it will get out in some other way.
If, on the other hand, the violence concerned isn't 'wanted', which usually means that people can't deal with it, the reaction will be either one of refusal to acknowledge it exists, or of becoming used to it. Action to stop it is only possible if somehow (some) people have the spare mental capacity to either take action (e.g., become mercenaries in Syria) or recommend taking action (e.g., they could pressure political leaders to go to war against Daesh).
As others have said, too much exposure to unwanted violence (or other unwanted phenomena, such as homeless people on the street) will usually result in denial or desensitisation. We can only do so much. If you take the WWI example, that was at the time the one issue going on and people were therefore very capable of forming a judgement upon being confronted with documentary evidence. However, had there been huge natural disasters or an invasion by Martians at the same time, that would have been very different.
There are two sides to 'violence'. On the one hand, things like boxing or other violent sports, e.g. cockfighting, sate a desire for violence, even vicariously, e.g. to be 'on the winning side' in a boxing match. If that desire isn't somehow given a vent in this way, it will get out in some other way.
If, on the other hand, the violence concerned isn't 'wanted', which usually means that people can't deal with it, the reaction will be either one of refusal to acknowledge it exists, or of becoming used to it. Action to stop it is only possible if somehow (some) people have the spare mental capacity to either take action (e.g., become mercenaries in Syria) or recommend taking action (e.g., they could pressure political leaders to go to war against Daesh).
As others have said, too much exposure to unwanted violence (or other unwanted phenomena, such as homeless people on the street) will usually result in denial or desensitisation. We can only do so much. If you take the WWI example, that was at the time the one issue going on and people were therefore very capable of forming a judgement upon being confronted with documentary evidence. However, had there been huge natural disasters or an invasion by Martians at the same time, that would have been very different.