WASHINGTON — Four days after his son died in Afghanistan, John F. Kelly, then a Marine Corps general, eulogized two Marines killed by a truck bomb in Iraq. He made only a fleeting, oblique reference to his son Second Lt. Robert Kelly as he paid tribute to the two other fallen service members who held their ground for six seconds as a truck bore down on them.

Now Mr. Kelly is President Trump’s chief of staff, and the commander in chief is testing his aide’s long-held reluctance to discuss his loss. Mr. Trump, in defending his handling of the deaths of four Green Berets in Niger, falsely claimed on Monday that President Barack Obama did not contact the families of fallen troops. And on Tuesday, Mr. Trump brought to light that Mr. Obama never called Mr. Kelly after the death of his son.

Such phone calls are not routine, especially when the rate of combat-related fatalities is high, as was the case in 2010, when Lieutenant Kelly was killed after stepping on a land mine while leading a platoon in Afghanistan. But Mr. Kelly is the highest-ranking American military officer to lose a child in combat in Iraq or Afghanistan.

Mr. Trump called the families of the soldiers killed in Niger, the White House said Tuesday, and the president has said that he would try to call as many families of American troops killed on his watch “when it’s appropriate.”