From North by Northwest, Alfred Hitchcocks greatest work.
From North by Northwest, Alfred Hitchcock's greatest work. YouTube

There's no need to call a detective with "daring perspicacity" for this case. This corpse has been cold for a long time, and the cause of its death holds no mystery. We can easily see what happened here. The UN's demise began with the decline of the nation that structured the institution to promote that nation's form of capitalism in a postcolonial world, and also to challenge its main statal rival, the USSR. Case closed.

But if you need a better understanding of the link between the UN as an institution and US's post-war hegemony, you only need read the highly readable book, The Long Twentieth Century, by Giovanni Arrighi, an Italian economist and historian whose life ended in 2008.

He writes:

After the Second World War, every people, whether “Western” or “nonWestern,” was granted the right to self-determination, that is to say, to constitute itself into a national community and, once so constituted, to be accepted as a full member of the interstate system. In this respect, global “decolonization” and the formation of the United Nations, whose General Assembly brought together all nations on an equal footing, have been the most significant cant correlates of US hegemony.

This institution, which is based in New York City for a good reason, idealized the American global market order. It was not going to be like the "old gang," the British one, which was fiercely colonial. In this new US system, the self-determination of nations was apparently granted. First, Second, and Third Worlds had, ideally, "equal footing" in the United Nations. There was even a rotating seat on the Security Council for poor countries. What more could you want?

But two developments led to the UN's loss of significance. One, of course, was the collapse of the USSR in the late 1980s, and the other was the arrival of the fourth "phase of accumulation" in a historical movement that began in the 17th century with Dutch capitalism.

A quick note: Arrighi's starting point for capitalism is, in essence, 15th-century Genoa. For Ellen Meiksins Wood, another gifted historical theorist, it's 18th-century United Kingdom. For me, it is 17th-century United Provinces. And the reason for this is found in a key aspect of capitalism that was first expressed here in the Lowlands: the mass production of luxury goods. (My position on this matter owes a great debt to another theorist of capitalism and its history, Noam Yuran.)

The USSR is gone. State socialism is dead. Russia is no longer a superpower. As for China, its form of capitalism has replaced in influence the American version, which entered its twilight around the time Ronald Reagan entered the White House and Wall Street became the world's financial center. (As the French historian Fernand Braudel once pointed out, the rise and dominance of finance in any global movement of value extraction marks the beginning of its departure from the main stage of capital accumulation.) And we can expect China, as the US did in the 20th century, to develop new tools of control for its own form of economic hegemony, which officially began, by the way, in 2008.

In short, it is China that matters when it comes to the future relationship between capitalism and carbon. Not the US, nor the UN, an institution that, unlike the IMF and World Bank, America failed to repurpose for its post-Bretton Woods/New Deal decline. What all of this means is that speeches being made by world leaders at the UN have little to no value. It is an empty institution for as long as it remains in New York City and is tied to moribund post-war objectives.

And so heard the man who leads the 19th century sequence of capitalism:

And the man who leads the 20th century sequence:

And the one who leads its 21st century sequence: