Here is a concise, recent and thoroughly orthodox historical judgment on the Nyon Arrangements of 1937:
There was one minor but significant exception to the inaction of of the British and the French. In 1937, pirates in the form of unidentified Italian warships, began sinking British and French merchantmen entering Republican ports. To this, a direct attack and affront, France and Britain responded with firmness. A Conference was called in September at Nyon, in Switzerland, and the law was laid down. The sinkings stopped abruptly. It was a most instructive incident, but no lessons were drawn. Indeed, the governments wished, apparently, to draw no lessons.
Here is another judgment, this one by a Soviet historian, which offers a somewhat different view of this episode:
Bourgeois political persons ignore the positive, determined role of the Soviet Union in the outcome of the conference and assign the achievement of its successful results only to the unity of England and France. [Thus] Eden, in a letter to Churchill written after the conference, explained that its results showed the wholesomeness and effectiveness of cooperation between England and France. “The two Western Powers proved that they could play a decisive role in European affairs.” Eden completely ignores the fact that at the conference in Nyon there took part not two, but three Great Powers. The third Great Power, which played a first-class role in the resolution of the problem of piracy, was the Soviet Union. Eden's statement does not answer the question, why earlier, before the Nyon conference, England did not succeed in attaining such results.