The demise of all fiat currency for hard money such as Bitcoin, Monero, or Gold, so that governments no longer can steal from the people to wage wars that kill millions upon millions of people would be a great start. It would not totally eliminate war, but it would drastically, drastically reduce it, because then governments would have to demand that people give them the money to execute the war, and if people did not want to do so, then they would not, and the government would have no say about that. Except for killing all their own citizens.