ROME — Prime Minister Prodi offered his resignation last night after his government lost a crucial vote in the upper house of parliament. The move plunged Italy back into the political instability that saw 50 different governments in the first 50 years after World War II.

Mr. Prodi lasted just 281 days after attempting to drive through tough and unpopular reforms despite only a two-seat majority in the Senate.

Yesterday, he lost a crucial vote on his government’s foreign policy, after his sprawling nine-party coalition divided over an increase in troops in Afghanistan and a decision to allow America to build a new Air Force base in Vicenza. Before the vote, the foreign minister, Massimo D’Alema, warned that the government would collapse if it lost. His words goaded the opposition, and some bipartisan lifetime senators, into staging a revolt.

As the results were read out, the Senate chamber resounded with cries of “Quit! Quit!” Mr. Prodi called a Cabinet meeting and offered his resignation to President Napolitano shortly afterward. A spokesman for the president said the president had not yet decided whether to accept it.

Mr. Napolitano is expected to hold talks with party leaders in the next few days to find a solution. He could allow the government to call a vote of confidence. The leader of the opposition, Silvio Berlusconi, said Mr. Prodi had no option but to resign, since a government loss on foreign policy “has never happened before in the history of the republic.”