It never seems to end. There is fighting in the Ukraine. Israel continues its ethnic cleansing campaign. Having separated from Sudan, now the peoples of South Sudan are fighting among themselves. Iraq, Somalia, Thailand ... the list seems almost endless.
On the other hand, a lot of people with different ethnic heritages, religions, and customs get along just fine, almost all the time. America has its famous melting pot, if you are willing to overlook the earlier genocide against Native American Indians and discrimination against non-Europeans (and Irish, Jews, Italians ...). Partly that is because we have created a new nationalist identity, itself dangerous to other nations of the world. America has a high-proportion of modern people, who see religion as mythology and ethnic identities as old-fashioned. In a way "modern" people are globalists. A modern person can find people with similar outlooks almost anywhere in the world.
Individuals often hold modern and old-fashioned contradictions within themselves. Woodrow Wilson, President of the United States from 1913 until 1921, was a racist, in fact one of the key intellectual architects of 20th century racial segregation. Perhaps because of his racism, rather than in spite of it, he proposed to the peace conference after World War I that people should have the right to "self-determination." National borders would be drawn by the people themselves, so that an ethnic group might choose to have its own nation, or to combine freely with other ethnic groups into a larger nation.
When Wilson and the American delegation got to Versailles they found there were a lot of people interested in self-determination. People in the British, French, German, Dutch, Belgian, Italian, and former Ottoman empires, for instance. Ho Chi Minh was there (think of all the trouble that could have been prevented had the conference granted Vietnam independence from France.) But none of the empires had any interest in granting freedom to their conquests, and Japan, France and the British Empire actually grabbed more territory. Being on the winning side trumped universal ideas of justice. Nor did it ever occur to Wilson to free Puerto Rico or the Philippines.
The Japanese asked that all Asian peoples be granted self-determination and freedom from (European) colonial domination. They also asked for a general provision against racism, which Woodrow Wilson personally nixed. The only new nations to emerge were carved out of the losers, Germany, Turkey, Russia and the Austro-Hungarian Empire.
Many of today's problems trace directly to the triumph of power and racism over justice at Versailles. Iraq, rather than being part of a larger pan-Arab nation, or being divided into Shiite, Kurd, and Sunni majority smaller nations, was created for British oil extraction. Palestine was divided off from Syria (which was given to the French) so that Britain could keep its promise to the Jews for their help in World War I. The nation now known as Jordan was given as a consolation prize to a pal of Lawrence of Arabia.
We cannot redraw the past, but modern people might well try to not repeat its obvious mistakes. I understand when ethnic groups and other minorities are, or feel they are, oppressed. That pushes them towards a desire for autonomy and independence. But nationalism, ethnic identification and racism are closely related. I think people do better when they trascend all that.
Adjusting borders in Africa, South America, and Asia that were drawn by European imperial powers makes some sense. This would have to be done on the principle of self-determination: let people draw their own lines. But many issues have no geographic solutions. Some minority groups have no majority areas. Many areas have complex mixtures of groups.
The better answer is to encourage the trend to a modern, friendly mindset based on the following principles:
Legal and economic justice for all.
Freedom of religion and expression.
Classic good governance: no corruption or favoritism of any kind.
Emphasizing our common humanity, not our cultural or physical differences.
Universal education in the generally accepted principles of ethics.
Human nature being what it is, setting things right will be a long, slow process. There have been several historical attempts to globalize an ethical system, usually a religion. I believe Christianity, Buddhism, and Islam all attempted to be universal religions, with a universal system of ethics for all people. Each failed, or at least has failed so far. Marxism (or more broadly, socialism including anarchism) also sought to be a universal cure for humanity. I think it is worth looking at each of the histories of these movements to see what they achieved and why they failed.
I believe the key is being universal, but not authoritarian. Universal tolerance and diversity can work as long as we share (and act on) core ethical values. Modern people do this. Modern people can treat anyone as an individual. Modern people do not oppress others. Modern people communicate and educate. Modern people show mercy. Modern people resort to self-defense only as a last resort, and never act as or facilitate aggressors.
There just are not enough of us, yet. But we can be found just about everywhere and anywhere. We just need to keep making more friends, and avoid corruption.