The remarks at 9pm (0100 GMT) will be Trump's first formal national speech on Iran from the White House since launching the US-Israeli war on Iran a month ago.
At the same time, Iranian President Masoud Pezeshkian sent an open letter to the American people ahead of Trump's speech.
“The continued aggression is affecting people's lives and attitudes,” Pezeshkian wrote.
The Iranian President added, “Despite all the pressure, we did not weaken; rather, our strength increased in many areas”.
“America launched an unjustified aggression against us twice during the negotiations…and what we are doing is a response based on a legitimate right to self-defence, not an act of aggression,” Pezeshkian said.
The White House has given few details on the address.
But Trump is expected to try to reassure the nation that US goals are being met and that he has a plan for completing the war, which has roiled the US economy and helped drive the 79-year-old Republican's polls into deeply negative territory.
Earlier Wednesday, Trump claimed a breakthrough, saying Iran's president was seeking a ceasefire, which was swiftly denied by Iran's foreign ministry.
Trump has said he can see the war winding down within three weeks, repeatedly insisting that the main objectives have been all but met.
Yet he has also threatened escalation, warning he could order targeting of Iran's civilian energy and drinking water infrastructure and even send in ground troops.
There has been especially mixed messaging from Trump on whether he will insist on Iran fully reopening the Hormuz Strait to oil and other commodity shipping or not.
As recently as Tuesday, Trump suggested this was not a key goal. However, on Wednesday, he said he would consider a ceasefire only when Hormuz is "free and clear."
"Until then, we are blasting Iran into oblivion or, as they say, back to the Stone Ages!!!" he wrote on his Truth Social platform.
Recent polling shows Trump's overall approval rating slipping below 40 percent, with disapproval climbing above the mid-50s as voters sour on both the war and its economic fallout.
Support for the Iran campaign itself is underwater, with majorities opposing the offensive and independents turning sharply against it.
The economic picture has compounded the problem. Gasoline prices have surged above $4 a gallon (over $1 a liter) for the first time in years, while consumer confidence has weakened, dragging down Trump's already fragile standing on the economy.
NATO rift
Donald Trump has rattled some of the United States’ closest allies, calling for a “reconsideration” of Washington’s membership in NATO after European countries declined to back the Iran campaign, a stance that risks compounding diplomatic damage already inflicted by his trade wars and threats to take over Greenland.
Markets, however, seized on more optimistic signals from the president.
Global equities rallied, and oil prices fell on Wednesday on hopes of a near-term end to the conflict. Brent crude was down about one percent at $102.85 a barrel, while stock markets from Asia to Europe posted strong gains.
Analysts cautioned that underlying economic risks remain acute, with oil prices still elevated and the Strait of Hormuz—a conduit for roughly a fifth of global supply—effectively shut.
Beyond the markets, critics say the strategic picture is increasingly difficult to reconcile with the administration’s rhetoric.
US and Israeli forces have struck thousands of targets and inflicted heavy damage on Iran’s military infrastructure. But the conflict has dragged on, Iran’s leadership remains in place, and the economic shock has spread globally, raising questions about whether Trump can deliver a clear endgame.
That uncertainty has reached Washington, where even some of Trump’s allies acknowledge the war is becoming a growing political liability ahead of November’s midterm elections.
A White House official said the president’s upcoming speech would highlight what it described as the success of the military campaign in achieving goals set out “prior to the operation”.
The administration says those goals include destroying Iran’s navy, dismantling its missile arsenal and production facilities, neutralising its militia proxies across the region, and ensuring Tehran can never obtain a nuclear weapon.
“He is expected to reiterate the two-to-three-week timetable for concluding the operation that he stated yesterday,” the official said.
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