Until now, research into the League of Nations economic discussions was a serious challenge. You needed to be an archives master like Mona Paulsen to make much progress.
But things have changed with a new digitization efforts by the UN folks, which includes most -- and eventually all, I think -- League of Nations documents related to economics and trade. It will still take some effort to navigate the searching of the very large PDF files of the scanned archive documents, but it is so much easier now to dig into the League of Nations work on the earliest multilateral trade talks than it was before this resource was available.
With these documents more accessible, here's a question that I am interested in, for anyone who is looking for a project to work on: What exactly was the thinking behind the move to go beyond tariff reductions as the core of trade agreements, and to shift towards the development of constitution-like principles such as non-discrimination?
Here's my very brief overview of how I think this played out, based on past research I've done.
Originally, trade negotiations focused on tariffs and quotas, which are the most obvious trade barriers, as they are imposed at the border. But the negotiators soon realized that domestic regulations and taxes could also be used for protectionist purposes, so bringing down tariffs wouldn't solve all the problems they were dealing with.
At the League of Nations, they spent a few years -- starting in 1927, but possibly a bit before that as well -- trying to categorize all of this "indirect" or "administrative" protectionism.
At the 1933 League of Nations economic conference in London, they went more deeply into this issue and came up with two solutions.
First, there was a suggestion to rely on the national treatment principle so as to prohibit most domestic regulations and taxes used for protectionist purposes.
And second, there was a suggestion for an "equitable treatment" clause that would require governments to act in good faith and not use domestic measures to undermine tariff concessions.
When there is a debate about which of two approaches to take, the final decision is often to use both, and that's what happened here: We got national treatment obligations and an equitable treatment clause (which was rewritten with the famous "nullification or impairment" language by the American delegate).
The national treatment obligation proved to be one of the core principles of the GATT/WTO. We can debate the proper interpretation (which we will no doubt do until the end of time!), but just about everyone agrees we need some version of this obligation.
The non-violation nullification or impairment remedy, by contrast, has only rarely been used, and its purpose and scope are very uncertain.
For anyone intrigued by all of this, a great place to start is the discussions of "indirect protectionism" (which the archives folks have separated out into a number of specific sub-series). From what I can tell, that was the primary way these issues were described at the time.