Now that the big five investment banks of America have been shut down, sold off or turned into more staid bank holding companies, there is one big risk-taking banker left: the federal government.

The decision this week to put $250 billion in banks in return for shares will make the government a major investor, and owner, in the banking industry. The bold move is one of many that is redefining and enlarging Washington’s role in the country’s banking system. Especially if conditions worsen, further government actions may be deemed necessary to shelter banks, businesses and consumers from the financial turmoil.

This fundamental shift at least raises long-term questions about government’s appropriate role in financing and the economy. Will the government inevitably be tempted to guide lending decisions, steering loans to some and not to others? History shows that government intervention in banking systems can carry its own dangers, with money funneled to political favorites instead of an economy’s innovators.

The federal government’s initiatives are all carefully cast as emergency measures that will be phased out in months or years. “It’s explicitly temporary, but we don’t really know where this crisis ends yet,” said Robert E. Litan, an economist at the Brookings Institution. “The logic of intervention is that the more ownership the government has, the greater the regulation and management control.”