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The ouster of WeWork’s co-founder and its botched initial public offering are a remarkable collapse of what was considered, up until weeks ago, one of the most valuable start-ups in the world.

This is not simply the failure of a young and capricious founder. Adam Neumann, who helped start the company in his early 30s, was a magnetic leader with a brash style that constantly invited controversy. The problem is that the adults in the room didn’t act like adults.

One of those adults was perhaps Mr. Neumann’s most critical enabler: JPMorgan Chase.

While SoftBank, the Japanese conglomerate, is the biggest and most notable investor in WeWork’s parent company, JPMorgan has been one of its most ardent backers for years, working multiple sides . It lent Mr. Neumann money personally (with his inflated shares as collateral), provided equity and debt for the company, served as a corporate adviser for the I.P.O. and secured nearly $6 billion in financing as part of the now scotched offering.

If there was one institution best placed to fully understand the various conflicts of Mr. Neumann, it was JPMorgan.