Education for everyone globally. War is, like all kinds of violence, an act or reaction of impotence (psychological term/not sexual).
It basically means nobody actually chooses to act violently or start wars. They do it because they believe consciously or subconsciously that they have no other options, because they can't think of any options.
This is always a wrong assumption, because there is always a better option. The difficult part is to getting people to understand their actual options. Education solves this.
No. I think Putin started the war because he was already threatened by the economy and domestic rivalry. It's not like he woke up one morning and thought it was a good idea to kill a lot of people. In his head it was the right thing to do for some reason, probably something involving saving his own ass.
He was able to gather some support for this horrible idea because there are many uneducated Russian voters who actually believe that the war has a purpose.
If they had been educated, he would not have had support.