You could look up the "Goulash Barons", (also known as "Minute Millionaires") in Denmark during WWI. Denmark, as a neutral nation state, experienced a raging economic boom during WWI due to food supply agreements with both sides of the war. Although "Goulash Barons" were typically seen as "nouveau riche" and looked down upon by the established upper class.

To try and visualize the actual magnitude of the economic boom: The number of factories that produced canned meat products exploded from a mere 21 to 148 over the course of the war, which was in response to an export demand 50 times higher than pre-WWI.

The Goulash in question was often made of the least valuable pig meat, entrails, tendons, behinds, legs and faces etc. And was pretty much made at lowest possible cost for highest possible profits. Ultimately Denmark earned the nickname "Germany and England's Breadbasket". Danish supply missions did end up costing 275 Danish ships, and 722 sailor fatalities.

Many did manage to acquire a considerable wealth, although some of the more prominent "Barons", like Harald Skovby Plum, ultimately backed the wrong buyer, in his case: the Tzar´s Armies in Russia, and consequently went bankrupt due to the Russian Revolution. He ultimately ended up killing himself in 1929 on his private island "Thorø".

After the war the "Goulash Barons" were openly frowned upon, being seen as little but two-faced profiteers of others misery. Today, you can find such relics for sale on Ebay as memorabilia plates with rude caricatures of "Goulash Barons", and other such nick-nacks.

If anything, the Danish profiteering off of WWI can serve as a supportive claim of Mr Butler´s essay, and although it may not have been a "racket", (racket sounding so subjectively derogative), the war certainly proved to be an immensely lucrative business opportunity for equally opportunistic businessmen.

I am a Dane, and this subject is actually commonly taught in history class at a Folk-School level. Also, please excuse any grammatical errors on my behalf, English is, as may be obvious, not my first language.

Edit: Typographical error.

Edit*: Changed Nuevo to Nouveau, thank you /u/grawlyx

Edit**: Expanded upon my response due to surprising amount of interest.

Anyone looking for my sources, or more literature on this subject and Denmark's involvement in WWI; (BE WARNED: Most source material is written in Danish):

Adriansen, Inge: ”Første Verdenskrig i Mikroperspektiv – Maria Gørrigsens åbenbaringer”, Fra Als og Sundeved, vol. 81 (2003).

Blüdnikow, Bent: Krigsfanger – et billeddrama om krigsfanger i Danmark under 1. verdenskrig (Odense, 1988)

Christensen, Chr. P.: Sønderjyder forsvarer Østafrika 1914-1918 (1937).

Christiansen, Niels Finn: Gyldendal og Politikens Danmarkshistorie, bind 12, 1900-1925, Klassesamfundet organiseres (2nd edit., Copenhagen, 2004), pp. 189 – 202.

Dinesen, Thomas: No Man’s Land (Copenhagen, 1929)

Dinesen, Thomas: Merry Hell! – a Dane with the Canadians (London, 1930)

Hansen, Ole Steen: Danmark under Første Verdenskrig (Holte, 2000)

Klos, Michael: Den 1. verdenskrig. Europas historie, 1870-1930 (2004).

Lindeberg, Lars: De så det ske, Danmark under 1. verdenskrig (Copenhagen, 1970s)

Sørensen, N. A.: ”Storbritanniens lange efterkrigstid, 1918-1945: Kultur- og politikhistoriske nedsslag”, Arbejdspapir no. 25. Centre for Cultural Research (Århus, May 1995).

Westh, Jan René: Danske frivillige i Frankrig 1914-1918/Volontaires danois en France 1914-1918 (1998)

When I saw the question I thought that it would most likely be that Smedley Butler got that figure from income tax returns. Sure enough, the sentence following the mention of "21,000 new millionaires and billionaires" clearly states that that was how "many admitted their huge blood gains in their income tax returns".

So Butler's figure comes from data that isn't limited to what you might think of as war profiteers. Rather, the data set is all Americans that filed income taxes. Butler calls this wealth accumulation "blood gains" but much of even the increased trade with the combatants would have been food and raw materials or other goods, with munitions being only a portion of it.

Butler's thesis is attempting to establish the motive that the elites intend to push countries into wars because they profit from them. However, what Butler's statistic actually shows is not that revealing, but is something that many economists have known for a long time: government spending boosts economies. If you took all the fighting, killing, destruction, oppression, hate, and sorrow out of warfare, all that would be left is massive government employment and spending. The military pays its soldiers wages and its supplier's invoices, and in turn, those soldiers and suppliers then have money to spend on goods and services, and that money spent goes into someone else's pocket, and then into someone else's, and so and and so forth. This is the multiplier effect of government spending. A dollar spent by the government, generates more than one dollars worth of economic activity beyond the direct initial payment.

In WWI, the US spent the majority of the war on the sidelines, exporting goods and lending money to friendly nations that were embroiled in the conflict. Even after joining the war, it was never exposed to the destruction that befell Germany, France, Russia, Austria, and other countries where the war was fought on their soil. So the impact of the war on the US economically was actually quite favorable. Not only did US businesses benefit from US military spending, but also in trade with other nation's whose governments spent heavily in their war efforts.

Of course, you don't actually need a war to get that kind of economic stimulus, just look at the US's modern military spending. In 2009, US domestic defense spending was 3.8% of GDP. In not one of the 50 states is defense spending less than 1% of the state GDP. In many congressional districts, the economy lives or dies by the defense jobs that their congressman protects from budget cuts. Obviously national security, preparedness, and relative power are all motivations for US defense spending, but I'm not sure what portion of the real reasoning are those and what portion of it is just a massive jobs program.

I don't think he was right, after all in 1918 there were an estimated 10,000 millionaires, at least according to The Politics of Rich and Poor by Kevin P. Phillips. He says that he got the information from tax filings, but there weren't 21,000 American Millionaires for years after he published his book. Not that he was wrong about a lot of the key points of US Marines being used to support American business interests or that the US created "Banana Republics" for the express purpose of letting US companies do business unimpeded by local concerns like fair wages and the personal safety of workers.

There's also a lot of money to be made in war... for those who aren't directly involved. A neutral party in the First World War could sell food to those nations whose farmland was being shelled fairly aggressively as part of a bid to break the fortifications that had been built across it. Yields still haven't recovered in some parts of France since the First World War, what with all the unexploded ordinance that is still being uncovered. So, lower supply but constant demand means higher prices for those still able to sell to the market unimpeded. Although, those aren't the kind of yields that turn farmers into millionaires overnight. Some people do better at the expense of those who are combatants.

For an interesting counterpoint to "war is profitable" you might want to look up The Great Illusion by Norman Angell. Originally published in 1909 he argued that future large wars would be futile from a purely economic sense because it was no longer feasible to pay your army on what you could get in a smash and grab job. You aren't going to bust down the door to a castle and haul off the crown jewels to pay your men. Winning costs tons of money, the only way to get that kind of money is to occupy farms and factories and let them run for a few years to recoup the expense. Only, that doesn't work out too well. People don't do so hot with the whole military occupiers taking their wages thing. Weimar Germany paid people to stay home rather than make reparations deliveries under the Versailles Treaty, for a period example. The war was hugely expensive to those who fought, England only paid off the last of its war debt last year. So, it's hard to say that war is profitable to for a whole people in the modern era.

Did people make a ton of money making and selling supplies and war materials? Absolutely. Was that more than they could have made otherwise? Doubtful. Remember, the war interrupted international trade. if you weren't making tanks then you would be making trucks and selling those instead. The millions dead in the war are millions of consumers that aren't buying trucks, lower demand means lower quantity supplied AND lower prices.

There definitely was a time when war was a racket. Smedly Butler was writing at the very end of that period when war became truly unprofitable.