When told by The Times that Mr. Obama writes the letters himself, she said the words became more powerful.
“It says to me that he, too, will be paying attention to more than just the numbers, but the real stories,” Ms. Merz said.
“One of the things I felt committed to even though I didn’t agree with our military ventures was reading the names of the troops killed as they were listed,” she said. “I just need to think about these people as individuals, and I hope that as a nation we are doing that and seeing them as real sons and daughters.”
She added that Cpl. Brennan was “just beginning to grasp the real life of the Afghanis around him, feeling very committed to the human side of what was happening in Afghanistan and Iraq.”
Their last conversation was on Inauguration Day, four days before Cpl. Brennan was killed by a roadside bomb. Both supported Mr. Obama’s election, but they spoke that day about the Afghan people, she said.
Ms. Merz said she had been considering making the trip from New York to Washington for the inaugural festivities, but “I’m glad I didn’t. I was home to get my son’s call.”
“I am so glad the president is trying to do the right thing personally in terms of families and the soldiers,” she said. “He has a monumental task in front of him.”
As Mr. Obama adjusts to the traditions bestowed upon the president as commander in chief, the duty is far different from saluting the Marine standing next to his Marine One helicopter or getting used to people standing when he walks into the room.
The president said on NBC News that the letters serve as a reminder - “that you’ve got hundreds of thousands of people - around the world who are putting themselves in harm’s way and you are the commander in chief.”
The White House said copies of the letters are preserved for historical archives.
The aide described the letters as “very gracious” but would not share the text because of the sensitive and personal nature of their content. The letters mention that the president appreciates the service member’s sacrifice.
While serving as a U.S. senator, Mr. Obama would send families of Illinois service members condolence letters and an American flag that had flown over the Capitol.
Some he would call personally.
Mr. Bush also sent personal letters to the families of every one of the more than 4,000 troops who have died in the two wars, he told The Times in an interview last year. He said he leaned on his wife, Laura, for support in the gut-wrenching task.
Mr. Bush also met privately with more than 500 families of troops killed in action and with more than 950 wounded veterans, said Carlton Carroll, a White House spokesman during the Bush administration.