People are differentiable into various categories, which means, unity for peace and tranquility requires interaction. Not just any interaction,
it requires honest and open-minded interactions.
One way to achieve it is through travel. Travel as often as you can. It really opens up your mind to accepting differences. Instead of finding faults, you encourage your mind to find similarities. You travel to a new place, you sense the difference of life lived by you versus the locals in that place. It brings to us fresh understanding that beyond the walls of the house we live in, beyond the conversations with our family/friends, and beyond the miseries/pains that we experience, there is another world that visibly looks like ours yet responds to similar situations differently. You open your heart and mind to new people, which resonates optimism. (https://www.smallerearth.com/uk/blog/9-reasons-why-you-should-travel-the-world/)
Another way is to stay curious. Curiosity is the key to knowledge and knowledge helps you evolve towards betterment. Curiosity is my strong character trait. I'm definitely not much of a traveler but I'm curious about the world. It helps me read, understand and learn lessons of life that keep me grounded. A curious mind's quest takes you through deep understanding of all kinds of emotions: defeat or success, hatred or love, betrayal or unity; if you start questioning your surroundings, actions and life members. For example, if I receive a rude reply for a nice gesture I did to someone, I debate it in my mind rather than express it upfront. A debate keeping both myself and that person's possible views helps me not get offended, and possibly understand the person better. I take offense (Newton's third law is unavoidable), but a curious mind helps me resolve conflict as I try to justify the action of the person, so reaction automatically changes.
Third way to bring peace and tranquility is by giving up ego. Having a 'I am not wrong' attitude or keeping yourself boxed in traditional belief system is the biggest hindrance to displaying rectitude in the world. However, if that is the choice of someone, Newton's third law, should still work on such narrow minds. They must be avoided at all costs. It is still debatable as to whether to leave them to realize it by themselves or make them realize it when possible. I choose my battles, that is, I only try to make those realize who matter to me. Others get their fair chance of giving up the unnecessary attitude, if I still find them rude because they have 'presumed' wrong intentions in my action, I become indifferent. I am still evolving and I am hoping to discover a better way to handle people with stubborn ego.
Alas, what matters is how much you ameliorate yourself in a lifetime. Peace will find its way out – before or after a war.