It's a pretty long and complicated topic there, it involves geopolitics too... If we look at Middle East we have sunni vs sciite, i highly doubt those two can live togheter in peace... But it would be too simplify reduce to that, nations are used by big powers for their proxy wars

Yemen devastated by civil war is nothing else but a proxy war of Iran on Israel, putting in Saudis in trouble too
Lebanon with hezbollah was an other proxy of Iran, now heavily weakened
Syria has just been used for a proxy war on Russia by US/EU/Turkey, with innocents dieing

I can go on forever, the point is, no one wants peace, big powers just play with smaller nations for their politics because if they wanted they could end any war