In comments, Jonathan asks the significance of the finding that Argentina is not alone in experiencing relative decline after 1914. Rather, it spans most of the Conosur. The fact that Uruguay also experienced decline is well known, so I will not comment more. And I will punt on Chile, since I know not sufficient economic history to say anything sensible ... there is evidence that Chile was in fact quite poor at the time. But I will discuss southern Brazil.
(Short version: southern Brazil was much richer than most think before WW1. It then experienced relative decline much like Argentina. Any explanation for Argentine decline that cannot also explain southern Brazil, or vice versa, is therefore likely to be wrong.)
Longer version: The popular version of Brazilian economic history is that Brazil in the 19th century was an underdeveloped country, like Peru. It then underwent a period of explosive catch-up growth in the 1950s and 1960s, but growth collapsed in the 1970s, only to pick up again recently.
That picture, however, does not jibe with what we now know of the non-economic history of Brazil or what we know of regional economic development in that country. Molly Ball (Rochester) has done some excellent work tracking down wage data in São Paulo. She collected data from textile firms, gas & electric companies, and the tramways. Ball’s textile wage data do not go back before WW1, but her series for the latter two industries go back to 1891.
It is possible to compare Paulista nominal hourly wages with U.S. wages for the same two industries, taken from Historical Statistics of the United States. (I calculated American hourly wages under the assumption that electric and tramway workers had a 48-hour week, which is consistent with data from the U.S. Postal Service; manufacturing workweeks fell from 55 to 50 hours in 1891-1913.) I could not easily find data on comparable Italian wages, but is also possible to compare them with the estimates of hourly wages for skilled construction workers generated by Stefano Fenoaltea (University of Rome Tor Vergata). The United States was, of course, the second-richest destination country in the New World; Italy was a major country of emigration in the early 19th-century to both America and Brazil.
Bearing in mind that these are nominal wages, the results look like this:
The basic image is that Brazilian wages were lower than in the United States, but much higher than in Italy. (They were within striking distance of much of northern Europe and beat Sweden.) It would be hard to call São Paulo underdeveloped the way we think of it today.
(The Brazilian volatility is almost certainly an artifact of the fact that the estimates come from industries dominated by monopolies in one city in whereas the American results are a national population-weighted average.)
Now, nominal wages are not real wages. It is possible that high prices in São Paulo meant that living standards were closer to Italy than to America. Brazil was highly protectionist at the time and had lousy internal transportation, so there are reasons to believe that many manufactured goods would have been pricier.
Fortunately, Luis Bértola and Carolina Román (Universidad de la República) have done much of the hard work. They find, rather astoundingly, that the cost of living in Brazil was one-third that of the U.K. before WW1, although it should be noted that their data is from Rio de Janeiro rather than São Paulo. Now, I strongly suspect that their estimates are off by a significant amount and the benchmark was Britain, not America ... but I would also bet that correctly accounting for the net effect of differences in the cost of living would still raise Paulista real wages with respect to the United States.
(I would also submit that for many migrants, who intended only to work and save in the new country for a while before returning home, nominal wages would be the relevant draw.)
In short, southern Brazil was rich before 1914. Not as rich as Argentina, not as rich as the United States, maybe not as rich as the richest parts of Europe, but rich.
The reason why Brazil as a whole ranked so poor in 1913 was that the United States of Brazil resembled the French Empire much more than it did an integrated nation-state like the Argentine Republic or the United States of America. Its huge northern hinterland, like France’s African territories, were populated by near-subsistence peasants isolated from the national economy and providing little save some tropical products. They were effectively prohibited from migrating south (or, in the case of France, north) by a combination of extreme poverty, poor transport links, and racial prejudice.
The problem is that a mass southward migration of northerners masks the relative economic failure of the south after 1913. We know much less about the timing of that decline than we should. We also know much less about the reasons for the decline than we should; unlike Argentines, Brazilians not only do not obsess about their fallen position in the world, they’ve forgotten that they ever had a better one.
The fact that the relative declines spans Argentina, Brazil and Uruguay means that Argentina-specific explanations are almost certainly wrong. Rafael di Tella may be correct that Argentina was “hit with 700,000 bullets,” but it can be asserted that the ones which did not also go to hit the neighbors did not kill the target.
Kind of long, but I wanted to make the point about Brazilian relative decline. Did I answer the question?