Over the past several days, as clouds darkened the sky over Arlington National Cemetery, familiar scenes played out: school children on field trips, tourists on guided tours and veterans wearing jackets and caps adorned with unit patches, walking in loose formations to visit military brethren lost in combat.

There have been at least 13 service members lost in the current conflict with Iran and it is unknown how many more may join the roll of the honored dead if a fragile cease-fire and potential peace deal fail.

The unknown dead of this and future wars has manifested at the cemetery, where an expansion is underway along the southern reaches of the grounds, adjacent to Section 60.

Far from the ceremonies of Memorial Day, Section 60 is where those lost in the Iraq and Afghanistan wars rest. The thousands of people who visit know the risks being faced by today’s military families through the loss they endured decades ago from another Middle East conflict.

A Son Lost to Iraq

Long before her son’s ashes were interred in Section 60, Sarah Vaughan thought of Memorial Day as just another three-day weekend, a calendar invite to head to the beach in the Tallahassee area where she grew up.

“Memorial Day was just a holiday. I knew the meaning of it, but I didn’t pay any attention to it,” Ms. Vaughan, 72, said in an interview from her Vail, Colo. home.

“But, boy, do I now,” she said.

Looking back, she said she realized that her son John S. Vaughan seemed destined for a military career. She remembered the schoolboy who sketched American flags into the corners of the school papers and always wore camouflage clothes around his hometown. Even his childhood TV favorites were The History Channel and The Military Channel, she said.

Ms. Vaughan described her son as “straight as an arrow,” doting on his younger sister Becca and looking after his single mother whenever he could. His independent spirit led him to hunt, thread his own lures for fly fishing and to get a pilot’s license.

John Vaughan joined the Reserve Officers’ Training Corps at Embry-Riddle Aeronautical University and after graduation entered the U.S. Army.

By June 2006, he was a 2nd lieutenant deployed to Mosul, Iraq, when a sniper ended his life on patrol at age 23, Ms. Vaughan said.

Now, Ms. Vaughan said, she will visit military cemeteries or memorials when she travels and tries to thank those in uniform for being willing to stand up for American values and freedoms.

“I think about it more than just on Memorial Day. I think about it a lot, and I’m just so proud of the bravery and the camaraderie these people have,” she said.

Ms. Vaughan said she prayed for those who remain in harm’s way and offered a piece of advice to their families.

“Stand up straight,” she said, “and be so proud of what their children are doing.”

Three Children Left Behind

More than 20 years ago, in 2005, Patty Stubenhofer spent Memorial Day searching for answers as she stood in Section 60 holding her three children in front of the grave of their father and her husband, U.S. Army Capt. Mark Stubenhofer.

Military service had been a big part of her upbringing. Her father served in the Navy; her grandfathers were in the Air Force and Navy; and her grandmother joined the Women’s Army Auxiliary Corps.

Yet, she said, she did not fully realize the commitment of military life until she was married, experiencing the months apart because of training or deployments. Mrs. Stubenhofer said she also learned the pride and love of a country born of those sacrifices. Two decades later, she said, it is still hard to put that feeling into words.

Mark Stubenhofer was killed on Dec. 7, 2004, in a firefight in Baghdad, Iraq.

“I am living the military family’s worst nightmare and it doesn’t take a conflict for them to become a surviving family,” she said, referencing training accidents and other non-combat deaths. “There’s nothing I can say that can prepare anyone for this.”

“I spent my first Memorial Day as a military widow holding the hands of our three children,” she said, “searching for the right words to explain the significance of this day.”

Ensuing Memorial Day weekends were not spent at graveside ceremonies, but with other surviving military families at the TAPS Good Grief Camp, she said.

During those weekends, her son and two daughters were paired with a mentor and placed in groups with other children to learn how to process their grief. Now, her children are in their 20s and have become mentors for the program.

“To us now, every day is Memorial Day,” she said. “It’s knowing that he loved us and his country so much that he was willing to stand on the front line and sacrifice his life to protect us and our freedom. ”

An industrial tank containing about 7,000 gallons of a highly flammable toxic chemical appears to have cracked, Southern California officials reported on Sunday. The development was interpreted as a possible sign that a catastrophic explosion or rupture might yet be averted as tens of thousands of evacuees waited to return home.

TJ McGovern, the interim fire chief of the Orange County Fire Authority, said in an update that firefighters conducted a “successful operation” on Saturday night to inspect the tank at a plant in Garden Grove that belongs to GKN Aerospace, a company based in the United Kingdom that manufactures aircraft components.

The container became increasingly pressurized on Thursday, heating the chemicals inside and releasing gas that could trigger an explosion. Firefighters responded, dousing the tank with copious amounts of water in an attempt to cool it. But GKN Aerospace’s team was unable to inject a neutralizing agent to reduce the chemical’s instability because of several failed valves.

“No one has ever had this situation before because the chemical is so volatile,” Chief McGovern said. He called the situation “unprecedented.”

The chemical inside the tank, methyl methacrylate, is used in the manufacture of resins and acrylic plastics, most notably plexiglass.

According to the Environmental Protection Agency, exposure to methyl methacrylate can irritate the eyes and skin and make it difficult to breathe, among other symptoms. Birth defects have appeared in animals exposed to the chemical.

On Saturday, local fire officials said the temperature inside the tank had risen more than 20 degrees and was still rising. By Sunday, it had reached at least 100 degrees.

There is fear of a “thermal runaway,” which could further generate heat, build pressure and cause a blast, said Elias Picazo, an assistant professor of chemistry at the University of California.

Alternatively, he said, a tank failure — in which the tank ruptures but does not necessarily explode — could lead to a controlled leak that could then be neutralized.

“I think the temperature within the tank has been steadily increasing and that’s indicative that the reaction is moving forward,” he said.

It is possible, officials said on Saturday, that the increase in temperature is occurring because the liquid inside the tank is solidifying. If so, and if the tank holds, that could make a rupture less likely.

A specialized team of officials from the fire departments in Los Angeles, San Bernardino County, Orange County and Long Beach were working on alternative solutions to prevent the tank from breaching, Chief McGovern said on Sunday. He did not provide details.

In a video posted to social media on Sunday, he said the team had found a potential crack in the tank, which might relieve some of the internal pressure.

“With this new information, it could change our trajectory and our strategy to this event,” he said.

Senator Thomas J. Umberg, a state legislator who represents the area, said that “several courageous firefighters” had discovered the small crack last night at about 8:30 p.m., after approaching the tank to adjust the water being sprayed on it.

The firefighters, he said, got close enough to the tanks to see that the internal temperature had hit at least 100 degrees, the maximum level that the gauges would register.

But no liquid was leaking from the crack, he said, which emergency responders interpreted as “a slight bit of good news.”

Mr. Picazo had said that the potential of the chemical solidifying would be an “ideal” but “unpredictable” outcome. “Then you have a lot of time to figure out what the best approach would be to open the tank and quench the remaining active material,” he said.

The fire authority said in another post that areas outside of the evacuation zone were considered “completely safe” and that daily activities could continue as normal.

​​Gov. Gavin Newsom of California declared a state of emergency in Orange County on Saturday. More than 40,000 residents in the surrounding areas are under evacuation orders, and officials have become increasingly concerned that some may be prematurely attempting to return home.

“We have a lot of citizens displaced and, when it’s safe to do, one of the things we want to do is to get them back in their homes,” Chief McGovern said in a phone interview.

Erika Ocana, who lives about a five-minute walk from the plant, evacuated on Friday with her four children, three dogs and a cat.

“I’m just thinking, like, what about the ones that are really close to it, what about the houses, what’s going to happen?” she said.

In a video posted to Facebook, Dr. Jason Low of the South Coast Air Quality Management District detailed the air measurements being taken in the community near the facility.

On Friday, the regional agency had begun measuring pollutant levels around the evacuation zone. Dr. Low said officials were “happy to report that levels are completely normal in our measurements.”

That agency has worked with the E.P.A. to deploy 24 monitors to continue the air measurements.

“We’re happy to report we have not seen any contaminants in those monitoring stations and we’ll continue to do that until the scene is secure,” said Harry Allen, an on-scene coordinator for the E.P.A.