For a long time prior to World War I (back at least to the Revolutions of 1847 and probably before), many Liberal* Europeans had thought that each ethnic group deserved self-determination, or the ability to manage its own destiny in its own homeland built along ethnic, linguistic, and cultural lines. Sometimes this involved unification: The Germans and Italians both found a national unity in the mid-19th Century and created nation-states out of a myriad of petty fiefdoms dating back to the collapse of the Western Roman Empire. In other cases, it involved separation, such as the Balkans repeatedly fragmenting and re-fragmenting, initially by destroying the Austro-Hungarian Empire after WWI and much later by destroying the Soviet Empire after the Cold War.
*(By Liberal I’m speaking in 19th Century European terms, which means Republicanism, Capitalism, and Rationalism, in opposition to Monarchy, Mercantilism, and (State) Religion.)
This notion of self-determination lasted well after the First World War, where it was a principle of redrawing the map of Europe, leading (among other things) to the independence of Finland, Estonia, Latvia, Ukraine, Lithuania, and Poland from the ashes of the Russian Empire. (The majority would fall under the dominion of the Soviet Empire, with Finland alone retaining a shred of quasi-independence.) The powers that be botched the Balkans (looking over the century, that is a defining theme) and created states that were not in harmony with self-determination, such as Czechoslovakia and the union of the State of Slovenes, Croats and Serbs (still alive October 29, 1918, not still alive December 1, 1918 (and you wonder why cartographers want to bomb the region off all of their maps)). This lead to a lot of trouble and is widely regarded as a bad move.
After World War Two, two things happened: The UN and the AK-47. Guess which one freed more former colonies. In any event, the concept lost a good deal of its luster with the invention of the Kleptocracy, the Murderous Kleptocracy, the Expansionist Murderous Kleptocracy, and the all-time favorite Single Expansionist Murderous Kleptocracy Seeking Nuclear Capability. Syphilitic strongmen who cook and eat their enemies has also tended to reduce the relative importance of self-determination in modern political discourse.
Like a sack of marshmallows, scorpions, and diamonds, self-determination is a mixed bag. I don’t know if India reached in quite as deeply as Europe did.