Western super powers were reluctant to give up their colonies. Great Britain had painted the globe pink with the extent of its empire and finally marched its armies out of India and other golden geese to retain them in the British Commonwealth and keep them as allies.
The consequences of just walking away resulted in horrific bloodshed in India. Muslims and Sikhs carried out atrocities on each other before finally splitting the country according to religious ethnicity.
Ireland waited a long time before self government was even considered. Westminster put off debating the Irish question and Catholic Emancipation until liberalism finally acknowledged that there was a case to answer.
Despite having it’s own parliament, Ireland was not a sovereign state and power remained in Westminster. Scotland was included in the Act of Union and is still campaigning for independence.
Sovereignty or self government remains an ambition for many countries. Vietnam waited hundreds of years to achieve that goal. Palestine is a conquered land betrayed by Western powers and its right to statehood thwarted at every turn by the Israeli government. Aborigines in Australia were treated like second class citizens and South Africa used apartheid to maintain white supremacy.
One hundred years after the Civil War in Ireland between pro and anti Treaty factions there is the possibility of a border poll. If there is a poll to determine whether there should continue to be a border between North and South the consequences of reunification will need to be considered in detail.