President Trump on Monday cast his plan for a cease-fire in Gaza as a landmark deal to bring peace after two years of catastrophic violence. But in reality, it was more like an ultimatum to Hamas.

Standing alongside Prime Minister Benjamin Netanyahu of Israel, Mr. Trump unveiled a proposal to which both men had agreed. If Hamas refuses to do the same, Mr. Trump said, the United States will let Israel “do what you would have to do.”

“Israel would have my full backing to finish the job of destroying the threat of Hamas,” said Mr. Trump, who under the plan would become the temporary chairman of a board in charge of the redevelopment of Gaza.

The joint appearance by Mr. Trump and Mr. Netanyahu at the White House was a clear display of unity at a moment when Mr. Trump has shown signs of frustration with the Israeli prime minister, and when much of the world has grown horrified at Israel’s prosecution of the war against Hamas in Gaza, which has killed tens of thousands of Palestinians.

But it was far from assured that Hamas would agree to their demands.

The U.S. plan contains provisions that Hamas has said publicly it will not accept, such as its removal from power and disarmament, leaving the proposal’s future uncertain and increasing the possibility that Israel will intensify its military campaign in the enclave, with the full support of the United States.

“When it comes to this plan, no one contacted us, nor were we part of the negotiations around it,” Taher al-Nounou, a senior Hamas official, said in a televised interview.