Clive Trebilcock's 1970 paper "Legends of the British armaments industry: a revision" is the seminal work here.

Trebilcock, who also wrote the standard history of Vickers (the British private arms giant which employed Basil Zaharoff, the arms dealer mentioned by u/Bodark43) turns the usual trope on its head by pointing out that governments actually hold almost all the cards in the state/arms company relationship. They are near-monopsonists – companies such as Vickers made significant efforts to build an export trade, but the sort of orders available from minor powers such as Chile or Brazil were dwarfed by the business available from a great power such as Britain, and for obvious reasons a British firm was never going to make substantial sales to rival great powers such as Germany or France. By maintaining competition among suppliers, such monopsonists can exert a substantial degree of control over cost, and hence the potential to profit. They can also insist on their suppliers maintaining an economically inefficient degree of spare capacity to allow to expansion of output in time of war. During periods of extended peace, they can cancel or withold orders, creating potentially catastrophic conditions for arms suppliers who rely on keeping a skilled workforce in full employment. Then, when war comes, they expect and require arms firms to suddenly massively increase output – yes, with guaranteed orders, but at their own expense – with little care or thought being given at a government level to what will happen to all that expensive plant and all those new workers when the war ends and the orders suddenly dry up overnight.

Trebilcock spent years planning a magnum opus on the pre-World War I arms trade which, sadly, he never lived to complete. Had he done so, he planned to title it The Perpetual Crisis, which sums up in a phrase the actual histories of major conglomerates such as Vickers, its deadly rival Armstrongs, John Brown and the like.

The trope probably came from the life and delightfully sordid career of Greek businessman Basil Zaharoff , ( 1849-1936) who travelled the world selling arms- notably the Maxim machine gun and the arms of the Vickers company- to various givernments using fear, bribery, sabotage of a competitor's product, and possibly on at least one occasion, shoving a competing salesman from a train. He would sell to both sides. Working for the Swedish industrialist/arms maker Thorsten Nordenfelt, Zaharoff marketed and sold one submarine to the Greek government, then warned the Turks that the Greeks had it, and sold them two- then warned the Russians that they could be shut up into the Baltic by the Greeks and Turks and sold them another two ( remarkably, Robert Fulton had tried something of the same trick with his submarine design, a hundred years earlier, marketing it to the French, British and Russians...but Fulton did not have a functioning submarine and in the end, only gained a British steam engine to export for his steamboat project in the US).

As Napoleon said, the only thing you can't do with bayonets is sit on them, so it's possible to say that arms dealers like Zaharoff made the job of diplomats much harder. Even Zaharoff can't be accused of actually starting a war, however. Although the arms dealers made the stakes higher and higher, it was still the European governments that started the first World War. His extensive sales to Spain did not result in the Spanish starting the Spanish/American War. He donated to the Greek government's disastrous attempt to take territory from the Turks, after WW1, but even there it's impossible to say that he supplied more than money and encouragement- there were existing national political interests that started it.

Ellis: The Social History of the Machine Gun.