The Spanish Civil War was an interesting case of the initially (industrially) inferior side eventually winning the war. Although Spain was rather undeveloped compared to the rest of western Europe, it did have some industry.

When the war did break out, the Republic managed to maintain control over the most important industrial regions - Barcelona and Madrid, as well as the mines in the north - controlling 60% of the country with 70% of its tax base as well as most vital supplies that were in the country at the time. However only 30% of Spain's agriculture fell into Republican hands at the outbreak of the war. Luckily much of this agriculture was in citrus and other cash crops, which the Republic could sell overseas for its much needed foreign cash and supplies. Even in mid 1937 after losing a large chunk of its initial ground to Franco, the Republic was concluded to be self sufficient still.

Unluckily for the Republic, though, they were plagued with disorganisation and internal revolution. With various parties fighting each other almost as much as Franco (especially the infamous May Days of 1937 in Barcelona), the Republic had to contend with little central control of the economy, as well as the large CNT collectivising industries and farms across the country (concentrated around Catalonia and Aragón) by its own accord and with variable success. With the rest of Republican government administration, the tax structure in the country crumbled with the breakdown of law and order in the initial outbreak, hindering their tax income efforts for at least the first year.

On the other hand, Franco managed to impose a fairly central and authoritarian economy, which had the benefit of foreign backing to keep it stable for the duration of the war, as well as the brutal methods the authorities used to keep the working population under control.

Another boost to the Francoists was their immense amount of foreign aid compared to the Republic. Almost as soon as the war started, Franco sought aid from Nazi Germany and Fascist Italy, which they both obliged with, particularly Italy which sent over 75,000 troops to aid the war effort over the 3 years of war. Franco received over $700 million (1940 USD) in aid completely on credit, compared to the Republican's $70 million from the Soviet Union. Furthermore, the Republicans had to sell their gold stocks to fund the rest of the aid from the USSR, which was the only nation willing to aid them due to the lack of willingness to intervene from France, America and Britain.

Francoist Spain managed to win the war despite their initial lack of potential in a war of attrition. Theoretically, the Republic should've won if one was to look at industry and manpower, but they didn't. It eventually came down to foreign support and internal stability, which Franco achieved much better.

Sources:

Seidman, Michael. The Victorious Counterrevolution

Preston, Paul. The Spanish Civil War: Reaction, Revolution and Revenge

Martín-Acena, Pablo; Martínez-Ruiz, Elena; Pons, María. War and Economics: Spanish Civil War Finances Revisited

another modern conflict you might add is the Russian invasion of Afghanistan. I think if both of the world's superpowers have lost modern wars to far less industrialized entities then what you suggest is hardly a "rule" of war.

You might also want to consider Portugal vs Angola, as well as the ALgerian revolution.

There's a few minor and debatable examples here and there; when Uganda invaded Tanzania in 1979, the Ugandans had an edge in terms of overall materiel, but were eventually ousted from Tanzanian territory after a counter-attack. The war was a short one, and the Tanzanians had great political allies plus a sympathetic 'hostile' civilian populace during their counter-offensive.

Another example, perhaps, can be found in the 1904-05 Russo-Japanese war. Technologically, the Japanese were more or less on par, due to a combination of local industry and import purchase, with their Russian foes and Western political allies, however, in most engagements, the Japanese were outnumbered and usually had inferior numbers of artillery pieces to the Russian foe. Yet they won; most put this towards a degree of overall skill and cunning versus relative Russian ineptness. Again, the war was relatively short, and the struggle was not to annihilation of a regime or state.

Its true that the longer the war lasts, the more pronounced the ability to replenish losses becomes a dominant factor - be they human or material losses. Most examples one will find tend to be between nations of approaching technological parity, if not production parity, and the victorious if less industrialized party will generally win with a series of knockout blows (as the Japanese did indeed try to do during WWII). Will to fight has a lot to do with it as well, if I could hazard a theory.

Can I add a follow up (slightly modified) question and ask if there are instances of established state-vs-established state wars (aka "Clausewitzian/Trinitarian" as vs counter-insurgency wars vs non-State actors/national liberation movements) that have been won from a production disadvantage?