I find it interesting how we are able to unite around an external enemy. Given a few generations, people start to forget that they were ever divided. We have short historic memories and seem to hop from battle to battle. When Muhammad arrived in the Arabian Peninsula, there were a bunch of tribes fighting each other. Armstrong argues that the key reform he brought was to let people be bound by faith, rather than tribe. Before that, law was enforced by vendetta. Mess with my blood, I'll mess with yours. Those tribes that now form the Arab grouping hated each other.
Shaka Zulu rose to power two years after, and far away from, the Battle of Waterloo. He brought together a bunch of very independent tribes over his 12 year reign from 1816 - 1828. The connection to Waterloo is that Britain suddenly looked down South to the power vacuum of the defeated Dutch (not the Boers, the ones they had tried to get away from) and sent some settlers. Interestingly Muhammad was invited to Yathrib (later called Medina) to form the ummah (a group based on Islam rather than tribes) just 10 years before his death in 632. He, and 2 years of Abu Bakr after him, united the tribes of Arabia - people who had been fighting each other for years. 12 years to form a nation. Conn Iggulden tells the epic story of Genghis Khan. He too, bound people who had hated each other. Who killed each other. Who feared each other. Who became each other.
The hatred gets forgotten years later. The Zulu 'nation' after Shaka was anything but harmonious and united. I believe the catalyst for change came with the Bambatha Rebellion. Struggling to recruit labour in the years after the Anglo-Boer war, the British imposed an onerous hut tax to encourage Zulu men to enter the labour market. They were not pleased and rose up. Together.
A lot of the fighting at the moment happens primarily between people who have a lot in common. Scott Alexander wrote a fantastic essay entitled 'I can tolerate anyone but the outgroup'. We seem much better at dealing with people who are VERY different from us. Looking at the past, it does however seem we are able (in the long run) to overcome differences. Khan, Zulu, and Muhammad were revolutionaries. Part of overcoming the conflicts in the past involved directing that aggression elsewhere. At some point, it would be great if the next revolution would be to not require an enemy.
Since, it seems that if history is a guide, one day that enemy will be family.