President Trump, even as a lame duck, is moving quickly to increase American sanctions against Iran and sell advanced weapons to its regional enemies, policies that would be difficult for a new president to reverse.

Last week, he asked his advisers for options to launch a military strike against Iran but appears to have been talked out of it. His aides argued that an attack could quickly lead to a larger war.

Iran, where President Hassan Rouhani faces strong opposition from conservatives in elections set for June 2021, is expected to demand a high price to return to the deal, including the immediate lifting of the punishing sanctions imposed by the Trump administration and billions of dollars in compensation for them.

Those are demands that Mr. Biden is highly unlikely to meet — especially given strong congressional opposition.