WASHINGTON — Prime Minister Benjamin Netanyahu’s vow for Israel to annex parts of the West Bank would flout four decades of American policy, under both Republican and Democratic presidents. But nothing emboldened Mr. Netanyahu to take such a risk more than the support of his ally President Trump.

From recognizing Israel’s sovereignty over the Golan Heights to moving the United States Embassy to Jerusalem, Mr. Trump has given Mr. Netanyahu the political cover and legal legitimacy to embrace a position that critics say would all but extinguish the dream of a viable Palestinian state.

Mr. Trump’s moves are not merely temporary gestures, which a future president or Israeli leader could reverse. They are policy changes that experts say have permanently altered the contested landscape of the Middle East, making his own stated goal of a peace agreement between Israel and the Palestinians more unattainable than ever.

“They talk about a peace plan, but we never see it,” said Martin S. Indyk, a former American ambassador to Israel who tried to negotiate a deal between Israel and the Palestinians during the Obama administration. “What we see are new facts on the ground that will make a two-state solution impossible.”