People have always employed sophistry of all kinds to justify their wars of agression. "Might makes right" has never gone out of style, though. It just evolves with the times.

If you look at Thucydides' History of the Peleponnesian War, you can really trace the collapse of Athens' moral authority. Athens' coalition of allies was nominally a system for equal states to pool resources for their mutual defense, a sort of ancient NATO. By the time of the Peleponnesian War, however, you can tell that their "alliance" has become a thinly-veiled pretext for Athens' imperial ambitions.

The devolution continues until with the Melian Dialogue, Athens brazenly admits that their strength gives them the right to lord it over the Melians, and impose whatever terms they like on a neutral city-state. Raw power meant everything then, and it still means a lot now.

I'm not entirely sure what you're asking, and to be fair I'm not really much of a historian, but if you mean to ask why conquering land in the name of a specific nation has for all intents and purposes stopped, then I can at least contribute a little bit to the answer. Essentially, this has stopped because the way that countries make money has differed somewhat in the past hundred years. Since World War I, which was essentially the end of traditional Imperialism seen since the 16th century, America has become the unrivaled leader amongst the rest of the world. This is because the state of the economy shifted from nationally-owned corporations like the British east- india company,for example, to a company like Apple or perhaps Lockheed Martin. This means that the state of the global economy is not reliant on one single country for the entirety of its profit. Why conquer China, when you could just sign a treaty guaranteeing no tariffs on production there? In short, the fall of 'right of conquest', is because of the global switch from Nationalism to Capitalism.