prop up the shares of GameStop, the video game retailer, determined to outsmart hedge funds that had bet the company’s shares would fall.

But what started as a spontaneous effort to take down Wall Street has since become an established presence in the market. Millions of amateur investors have embraced trading, including more sophisticated strategies such as shorting. As the Credit Suisse incident shows, their actions highlight a new source of peril for troubled companies.

Founded in Switzerland in 1856 to help finance the expansion of railroads in the tiny European nation, Credit Suisse has two main units — a private wealth management business and an investment bank. However, the bank has often struggled to maintain a pristine reputation.

It has been the repository of funds from businesspeople who are under sanctions, human rights abusers and intelligence officials. The U.S. government has fined it billions of dollars for its role in helping Americans file false tax returns, marketing mortgage-backed securities tied to the 2008 financial crisis and helping customers in Iran, Sudan and elsewhere breach U.S. sanctions.

In the United States, Credit Suisse built its investment banking business through acquisitions, starting with the 1990 purchase of First Boston. But without a core focus, the bank — whose top bosses sit in Switzerland — has often allowed mavericks to pursue new revenue streams and take outsize risks without adequate supervision.