President Biden paid tribute to veterans who died in America’s wars at Arlington National Cemetery on Monday, hailing them as “a link in the chain of honor” who deserve recognition for protecting the nation’s democracy.

“Freedom has never been guaranteed,” Mr. Biden said in a nine-minute Memorial Day address, moments after placing a wreath at the Tomb of the Unknowns.

“Every generation has to earn it. Fight for it. Defend it in battle between autocracy and democracy,” he said of the nation’s veterans. “Our democracy is more than just a system of government. It’s the very soul of America.”

His somber message was a sharp contrast to that of former President Donald J. Trump, his challenger for re-election this year, who posted an angry and incendiary Memorial Day message on his social media site.

“Happy Memorial Day to All, including the Human Scum that is working so hard to destroy our Once Great Country,” the former president wrote on Truth Social.

Mr. Trump also wished a happy Memorial Day to Lewis Kaplan, the federal judge who oversaw the trials in which the former president was accused of defamation. Mr. Trump called Judge Kaplan “the Radical Left, Trump Hating Federal Judge.” He also called Justice Arthur F. Engoron, who presided over Mr. Trump’s civil fraud case, a “wacko.”

In an earlier, more traditional Memorial Day missive on Truth Social, Mr. Trump posted a photograph of himself saluting a wreath while he was president, and saying “WE CAN NEVER REPLACE THEM. WE CAN NEVER REPAY THEM. BUT WE CAN ALWAYS REMEMBER.”

But his vitriolic post followed a few minutes later as a reminder of the stark differences between the two rivals for the White House. As has been the case for years, either as president or as a presidential candidate, Mr. Trump showed that he would not be held to the norms of behavior that guide the nation’s leaders on a somber national holiday.

By contrast, Mr. Biden’s appearance was typical of the kind of message delivered by other presidents in both parties.

He focused mostly on the sacrifices of the past — members of the military who died in Afghanistan, Korea, Vietnam and the two world wars.

“Our fallen heroes have brought us closer today,” he said. “We’re not just fortunate heirs of their legacy. We have a responsibility to be the keepers of their mission, that truest memorial of their lives.”

Mr. Biden did not mention the wars raging in Europe and the Middle East, where he has pledged not to send American service members to fight alongside allies in Ukraine or Israel. The United States has been drawn into both conflicts nonetheless. The military is helping to provide humanitarian relief in Gaza and is equipping and supporting fighters in both places against Russia and Hamas.

Mr. Biden took a moment during the speech to remember his son Beau, who died of brain cancer after serving in Iraq as a member of the National Guard. The president has long said he believes his son developed his cancer as a result of living next to open “burn pits” in Iraq, where the military would burn waste, producing toxic smoke that nearby soldiers breathed in.

“This week marks nine years since I lost my son,” the president said, being careful to add that Beau Biden’s death was “not the same” as those of soldiers who lost their lives in battle.

”The pain of his loss is still with me every day as it is with you,” he said. “Still sharp, still clear. But so is the pride.”

It is that pride, the president said, that Americans must remember when service members return home from battle, or when their grieving family members receive their remains.

Mr. Biden has repeatedly said he believes that the country’s “most sacred” obligation is to take care of the military members who return home from battle, or to care for the relatives of those who have died. On Monday, he repeated that pledge again.

He hailed the passage in 2022 of legislation that he championed to make it easier for service members to receive health care and other benefits for ailments most likely caused by burn pits and other toxic exposures.

Mr. Biden said the legislation and other efforts to support veterans was an expression of gratitude from a thankful nation.

“Gratitude to our fallen heroes,” he said. “Gratitude to the families left behind. And gratitude to the brave souls who continue to uphold the flame of liberty all across our country and around the world.”