I'm actually seeking the exact timeframe or transition of perception from when conquering nations once considered "powerful/great" started to be realized as "oppressive/tyrannical". Essentially when was it that society as a whole started to scrutinize nations and hold them accountable instead of glorifying their despotism.
Hi! I study cultural history and this is actually a super interesting question, to which I simply have no clear answer, but a rather vague one. I see you want an exact timeframe, I will argue why that is practically impossible, but I will try and specify it for you as exactly as I can.
It’s essentially about Zeitgeist, which I’m going to grossly simplify to the feelings and beliefs in a certain time period. (Neo)colonialism is not inherently good or bad, nothing is. So what you want to look for as a historian is the paradigm shift in opinion about it. The hard part of this is first of all location, Zeitgeist is not unanimous in all of the world, Vikings might have different ideas on colonization than the Mongolians. That’s why you can’t really put a mark on a calendar, but let’s assume we are talking global opinion here.
This also means that societies always had to ‘know better’ the ‘knowing better’ was just a product of their own zeitgeist. This absolutely does not mean things like slavery are excusable, from various cases we know that slaveowners/trades also knew that they were being morally wrong.
Now the second tough point is significance. So we are essentially looking at forms of protests and public opinion about colonization, when are those significant enough for you to qualify as a the ‘general’ opinion or vision?
So I’m going to assume that you mean: When did the global public opinion on colonialism shift? So lets place it on a timeline.
In the late 18th century there were already revolutions and revolts going on (think of Haiti) for various causes and reasons, yet this was usually by stakeholders and not really a massive public opinion.
I think a very clear example of a debate were many were involved was the global debate around slavery. Here people are more actively taking on our current day zeitgeist. Which gained traction in more various forms during the 19th and 20th century.
From 1945 onward the floodgates for anti colonialism opened for various reasons, the 1960s would later mark enormous human rights movements, which because of globalization made a huge impact.
You can say something for all these dates, that’s the beauty of history.