How do we achieve peace between nations?
Two things come to mind.
When we say we don't want war we must mean it... we must be real pacifists.
We must earn our living without taking from other nations greedily.
One thing that should be noted is that there is a high number of nations. The more nations there are, the greater the odds that one or more of them will have a dispute which leads to war.
It might be easier to maintain world peace and order if there were only a few large superpowers, with no small fry to complicate things.
The prevailing notion nowadays is that every nationality, however small, should have their own separate, sovereign territory which should be considered sacrosanct. This is a nationalistic ideal, and nationalism (along with its offshoots, tribalism and racism) often seems a leading factor in causing a lot of these wars.
Of course, I'm not negating the economic factors, and the vast disparities between the haves and have nots also remain as causes of wars, revolutions, and dissent.
Also, it's not really so much a question of peace "between nations," since that's only one part of the equation. Much of what we see nowadays is war and/or insurrection within nations. That seems to happen more than wars between nations.