"Long ago, far, far away," the church used to think of the Great Commission ('go into all the world, make disciples of all the nations') in terms of countries or nation-states.
After Winter's 1974 address to the Lausanne Congress on World Evangelization, that began to change. The mission enterprise began to focus more on peoples, and specifically on unreached peoples.
A people specific focus is harder. The boundaries of countries are (mostly) clear. People group boundaries are not always so.
These boundaries affect not just the spread between peoples (e.g. between Japanese and Koreans). When a group is large enough, you can't reasonably speak of reaching 'all' of the group with one strategy. There are boundaries inside the people group, between individuals.
Reaching Istanbul does not automatically reach Ankara; reaching urban cities does not automatically reach rural villagers; reaching college women does not automatically mean reaching housewives, who in some cultures are very secluded.
This is the important value Winter was getting at in '74: segments within a society are cut off from the Gospel because the choices we, who have the Gospel, are making about which languages we use, what methods we use, who we work with, where we work.
Just because we are now focused on peoples, not countries, doesn't mean we are actually 'winning.' We can fall for the same problem within peoples that we fell for within countries. We can reach everyone in Istanbul... in a way that does NOT multiply out to the rest of Turkey. We can reach everyone in Beijing in a way that does not extend the Gospel to Xinjiang. We can reach everyone in Jakarta in a way that does not get to the other islands of Indonesia. And this is the exact same failure.
Our choices can limit who hears the Gospel, and thus "in a sense" our choices--or lack thereof--determines the eternal destinations of some.
(There's a ton theologically in there, and you might not agree with it--on some days, I don't agree with it--but nevertheless, whatever we think, we need to be very mindful of the eternal implications of our strategies.)
The task Jesus gave us isn't as simplistic as 'plant an outpost community within each large group' be it a country, city-state or ethnic group. This reduces 'make disciples of the nations' to its minimum: 'I am satisfied if you make a few (perhaps 12?) disciples of the group, no matter how large' equals "make disciples of every nation" (he didn't say how many disciples...).
I believe Jesus' heart for the world (read John 3) demands we be thinking about how to aim for the maximum, not the minimum, completion of the task. We need to think about how to disciple the whole world. We need to think about how to invade every last, tiny segment with the Good News: not just the pie, but every ingredient of the pie.