U.S. President Donald Trump salutes after speaking to over 1,110 cadets in the Class of 2020 at a commencement ceremony on the parade field, at the United States Military Academy in West Point, N.Y., Saturday, June 13, 2020. AP

As the nation continues to grapple with its racial past, President Donald Trump urged West Point's graduating class Saturday to ''never forget'' the legacy of soldiers before them who fought a bloody war to ''extinguish the evil of slavery.''

Trump's appeal to remember history came as his own relationship with the military is under strain from the unrelenting criticism he and Pentagon leaders have faced over their response to protests that erupted after George Floyd's death in Minneapolis.

It also came hours after Trump made what amounted to a rare concession for him: He rescheduled a campaign rally planned for Tulsa, Oklahoma, on June 19. The day marks the end of slavery in the U.S., and Tulsa was the scene of a fiery white-on-black attack in 1921.

''What has historically made America unique is the durability of its institutions against the passions and prejudices of the moment,'' Trump told more than 1,100 graduates at an unusual outdoor ceremony held during a pandemic. ''When times are turbulent, when the road is rough, what matters most is that which is permanent, timeless, enduring and eternal.''

In the past two weeks, Trump has yelled at Defense Secretary Mark Esper for publicly opposing his call to deploy active-duty troops to quell the protests stemming from the killing of Floyd, who was black, by a white Minneapolis police officer.

Trump also shut down Esper's attempt to begin a public debate on removing the names of Confederate Army officers _ some of whom trained at West Point _ from military bases, an idea gaining momentum across the country.

Gen. Mark Milley, the Joint Chiefs of Staff chairman, risked Trump's ire Thursday by declaring he had made ''a mistake'' by accompanying Trump on a June 1 walk through Lafayette Square. It ended with the president posing with a Bible outside a boarded-up St. John's Church.

Milley's comments amounted to an extraordinary expression of regret by Trump's chief military adviser, who said his appearance led to the perception of the military becoming embroiled in politics, which in his view _ one shared by Esper _ is a threat to democracy.

The events have stirred debate within the military and among retired officers. More than 500 West Point graduates from classes spanning six decades signed an open letter reminding the Class of 2020 of its commitment to avoid partisan politics.

The letter, published this week on Medium, also alluded to the problems Esper and Milley encountered at the White House after Floyd's death.

''Sadly, the government has threatened to use the Army in which you serve as a weapon against fellow Americans engaging in these legitimate protests,'' they wrote. ''Worse, military leaders, who took the same oath you take today, have participated in politically charged events. The principle of civilian control is central to the military profession. But that principle does not imply blind obedience.''

During the commencement ceremony, protesters denounced the president from boats and kayaks along the nearby Hudson River.