For conventional business and trade, that is true. We're talking about war profiteers, a different type of businessman who profits from the churn.
(Genuine question, I honestly have no idea)
[1]: https://foreignpolicy.com/wp-content/uploads/2019/01/dod_cha...
That's less revenue for an entire industry than Apple or Walmart does per year.
So how exactly are you proposing that less than 6% of the economy is directing the other 94%. Because that doesn't make any sense. For that matter, if war is so profitable and defense such a good market, why have so many companies which used to have major defense business exited that business? In WW2 just about every big business was partaking in the war effort. Companies like Ford Motors was producing aircraft and tanks. Ford has long left defense, and what remaining companies are in the sector have been consolidating or dying. Not exactly the sign of a booming industry.
Wall Street banks (especially JP Morgan) had loaned huge amounts of money to England and France. They stood to suffer masses losses if the war didn't turn out favorably for them. On April 24th, 1917, only 8 days after the US entered the war, congress passed the Emergency Loan Act which sent hundreds of millions of dollars to England and France. The lion's share of this money was used to immediately make interest payments to JP Morgan.
>Morgan brokered a deal that positioned his company as the sole munitions and supplies purchaser during World War I for the British and French governments, bringing his company a 1% commission on $3 billion ($30 million). He was also a banking broker for financing to foreign governments both during and after the war.
>Morgan played a prominent part in financing World War I. Following its outbreak, he made the first loan of $12,000,000 to Russia.[6] In 1915, a loan of $500,000,000 was made to France and Britain following negotiations by the Anglo-French Financial Commission.[7] The firm's involvement with British and French interests fueled charges the bank was conspiring to maneuver the United States into supporting the Allies in order to rescue its loans. By 1915, when it became apparent the war was not going to end quickly, the company decided to forge formal relationships with France.
https://en.wikipedia.org/wiki/J._P._Morgan_Jr.#World_War_I
Then there were the manufacturers and the farmers. Demand for goods, foodstuffs and merchandise of all kinds was a large part of what helped pull the moribund US economy out of a long period of economic distress that stretched all the way back to The Panic of 1907. England and French (borrowed) money was flooding into the coffers and US companies and US workers. A poor outcome for England and France would leave an uncertain future (at best) for the prospect of future exports for many companies.
There were other, non-economic factors to be sure, but much has been written about the influence that the banks and the business community had on pushing US entry into the war.