Lex Greensill portrayed himself as a savior for small business.

He started Greensill Capital to give the little guy a banking service mostly reserved for blue-chip companies: supply-chain finance, a type of cash advance that helps when payments are due from customers.

Mr. Greensill, the son of an Australian melon farmer, wanted to bring this helpful service to millions of smaller, less-established businesses. He planned to build a technology platform that would outrun bigger competitors such as JPMorgan Chase & Co. and Citigroup Inc.

His world came crashing down this week when Greensill filed for bankruptcy, ensnaring a global network of borrowers—more than half of them in the U.S.—as well as the firm’s financial backers, SoftBank Group Corp. , Credit Suisse Group AG and Japanese insurer Tokio Marine Holdings Inc.