The first world war was inevitable because society was begging for it.
In the century before, people were moving from the farms and into the city. People saw themselves as getting weaker, and saw war as a way to prove their strength. They glamorized it, recounted all the great military victories. They thought another great war would be a feather in their caps, and that it would only last 3 weeks tops.
They were wrong.
Stopping the death of one man won't stop the war. They would have simply found another excuse to fight. The alliance with Germany however... They guilt tripped Germany into backing them, pointing out they had an alliance with Germany, and Germany had never before really supported them. Germany was desperate for allies at that moment in time, and so they gave 'the blank check', which allowed the war to escalate to such a degree that it did.
Technology was ALREADY developing. It was already making war far more devastating than what man could imagine. If you could stop the alliance with Germany from bringing them into it, and keep that particular conflict to just those nations, then perhaps you could use that one war to show people how things have changed. Based on how stubborn people can be, even this is doubtful, given how anyone opposed to war was call a coward at the time. But perhaps...
The world wars were a game changer. It wasn't the wars that brought the technology, that was already being developed. But people saw how much more deadly it could be. Prior to these wars, the kill count was proportionally higher. And prior, technology had been used to attain even higher proportional death tolls. After this point in history, the proportional death toll begins to decline in war. We changed.
What bugs me is when idiots like Bush jr say stupid things like 'This generation has never been tested', as if we NEED to go out onto the battle field. It was promising that we weren't tested in battle! It showed we were growing, that we could resolve conflicts without violence.