By Stephen Collinson, CNN

(CNN) -- Donald Trump will ask Americans Monday to trust him on his new Afghanistan strategy, exercising a president's most somber duty, a decision on waging war, at a time when his own political standing is deeply compromised.

Trump will make his first prime-time broadcast on a specific policy issue to the nation as president at 9 p.m. ET to unveil his new plan, and a potential escalation of the nation's longest war, after a lengthy period of deliberations that carved deep splits within his administration.

The speech will test the President's capacity to convince Americans that he has settled on the right course of action on a major national security issue, and to unify the nation around it, despite his own depleted approval ratings and behavior that has alienated many voters in his first seven months in office.

Trump's first major national security address will also begin to show whether the credibility that the President has squandered, with his provocative rhetoric and frequent resort to falsehoods, will hamper his capacity to lead.

Monday's address, from Fort Myer, in Arlington, Virginia, represents a chance for Trump to leverage the symbolism of his office to stabilize a presidency that has threatened to spin out of control over the last two weeks.

Defense Secretary James Mattis said Sunday that Trump's policy review on the war and the entire US approach to South Asia -- i.e. Washington's tortured relationship with Pakistan and complaints that Islamabad is tacitly encouraging extremists -- had been finalized.

"He wants to be the one to announce it to the American people," Mattis said. "He now needs the weekend to collect his thoughts on how he's going to explain it to the American people."

While Trump will be unveiling a crucial national security decision, it will be impossible to divorce his speech from its political context. His inflammatory news conference last week in which he equated white supremacists and counterprotestors who clashed in Charlottesville, Virginia, triggered a stampede away from the President by senior Republican lawmakers, corporate CEOs and others that left him more isolated that ever before. His remarks also hit his own bottom line -- a long list of charities has now canceled plans to host events at his Florida resort at Mar-a-Lago.

The decision on Afghanistan also sets up a test for Trump with his own political base in the wake of the departure of his senior strategist, Steve Bannon, who opposed sending more troops to the war and was the closest link to the isolationist, populist beliefs of the President's core supporters.

Trump's long-awaited Afghanistan strategy

Trump repeatedly questioned the purpose of America's continued involvement in Afghanistan during his campaign, but he also contradicted himself on whether the war should have been fought.

Officials say that the President remains deeply skeptical about the notion of a continued presence in Afghanistan but is concerned that if the US comes home, it will leave a vacuum that could be exploited by terror groups.

But his doubts about escalating the war come up against the determination of hawkish generals in his inner circle, including national security adviser H.R. McMaster, not to give up on a war that has demanded more than a decade-and-a-half of painful American sacrifice.

Trump delegated authority to adjust troop levels to Mattis early in his administration, but he has been presented by the Pentagon with a range of options for the path forward, including a complete troop withdrawal and the deployment of up to 4,000 more soldiers to add to the more than 8,000 American forces that are already there.

Hopes that the US could finally leave Afghanistan have been checked by the Kabul government's struggle to preserve order under a resurgent challenge from the Taliban and inroads made by extremist groups like al-Qaeda and ISIS.

In June, Mattis gave a blunt assessment on the state of the war in a hearing for the Senate Armed Services Committee.

"We are not winning in Afghanistan right now. And we will correct this as soon as possible," he said.

Trump's appearance Monday night follows the most polarizing chapter of a presidency that has continually exacerbated political divisions.

His handling of the aftermath of violence at a white supremacist rally in Charlottesville earlier this month has sparked outrage and accusations that Trump has irrevocably tarnished the moral authority of his presidency.

A measure of the damage that his conduct has inflicted on his political fortunes is reflected in the continued ramifications of his news conference days afterward. A flood of CEOs distanced themselves from Trump, forcing the closure of several White House advisory councils. Some senior Republicans, including Sen. Bob Corker of Tennessee, publicly questioned his fitness for office. And few senior GOP figures have been willing to publicly defend Trump, raising new questions about his capacity to enact his agenda in Congress.

Speech, then rally

Should Trump succeed in staking out a more conventional presidential posture Monday night, any political gain could be short-lived, as he is scheduled to hold a major political rally in Arizona on Tuesday night.

A vintage, pyrotechnical performance by Trump could revive the issue of his temperament following the Charlottesville drama and further anger Americans who are already disgusted by the President's antics.

The rally offers Trump an early chance to show that despite the departure of Bannon, he remains committed to the issues and the political style that has won him steadfastly loyal support from his political base.

The issues likely to please a fervently pro-Trump crowd in Arizona are those that proved most effective for Trump in his campaign -- including on illegal immigration and the need for a border wall -- but which also cause the most alarm outside his core supporters.

In many ways, the Charlottesville episode has offered a reminder of why Trump was so controversial -- by tearing at the societal and racial divisions in American life -- but also why he was elected, since it shows how he connects with the often unspoken political instincts of a sizable slice of the country.

But in two crucial days for his administration Monday and Tuesday, Trump has to navigate two sides of his position -- the duty to rally a nation behind a foreign war and a political imperative to solidify support among his most enthusiastic voters. It is not clear that either aim is compatible with the other.