WASHINGTON — The chief executives of the nine largest banks in the United States trooped into a gilded conference room at the Treasury Department at 3 p.m. Monday. To their astonishment, they were each handed a one-page document that said they agreed to sell shares to the government, then Treasury Secretary Henry M. Paulson Jr. said they must sign it before they left.

The chairman of JPMorgan Chase, Jamie Dimon, was receptive, saying he thought the deal looked pretty good once he ran the numbers through his head. The chairman of Wells Fargo, Richard M. Kovacevich, protested strongly that, unlike his New York rivals, his bank was not in trouble because of investments in exotic mortgages, and did not need a bailout, according to people briefed on the meeting.

But by 6:30, all nine chief executives had signed — setting in motion the largest government intervention in the American banking system since the Depression and retreating from the rescue plan Mr. Paulson had fought so hard to get through Congress only two weeks earlier.

What happened during those three and a half hours is a story of high drama and brief conflict, followed by acquiescence by the bankers, who felt they had little choice but to go along with the Treasury plan to inject $250 billion of capital into thousands of banks — starting with theirs.