WASHINGTON — At a time when the U.S. military has the highest number of parents among its active-duty service members and is engaged in the longest sustained military conflict in history, in Iraq and Afghanistan, new research is showing that the strain on military families is being felt acutely by even its youngest members, children under the age of 6.

Young children can exhibit the same anxiety, depression, stress and aggression that some older children and adults experience after living with multiple deployments, long separations, and often tense and awkward reunifications with parents returning from war, particularly when the parent has been physically or mentally traumatized.

A new report released Monday by Child Trends, a nonprofit research center, found that while children are resilient, war can take a steep and potentially long-lasting toll during a child’s critical early years, when the brain is growing rapidly and children are developing a sense of trust in the world.

“We’re concerned that children exposed to stressful events, particularly traumatic stressful events, will have difficulty learning to cope with emotions, to do well socially and academically, and even have problems with their physical health,” said David Murphey, Child Trends researcher and report author. “As these younger children grow up, we can expect there will be at least a subset of them that will face very substantial problems.”

Since little research attention has been given to young children, Murphey said, many parents don’t understand why their children act out and often respond with anger, which serves to ratchet up family stress levels.

The report recommends educating parents and caregivers to better manage child behavior, providing greater mental health support for families and ensuring high-quality child care for military children — something Murphey says is a challenge when two-thirds of military families live outside the supportive environment of military bases.

Unlike during the Vietnam War, when only 15 percent of active-duty military were parents and most of them were men, today, nearly half of all active-duty service members have children, 14 percent of those service members are single parents. Mothers make up 16 percent of the active-duty force. Two million children under the age of 18 have an active-duty parent, and 500,000 of those children are under the age of 6.

The report, “Home Front Alert: The Risks Facing Young Children in Military Families,” a survey of scientific literature over the past decade, notes that stress levels for military families are unprecedented.

Nearly one in five service members returning from Iraq or Afghanistan reports acute stress, depression or anxiety, including high numbers diagnosed with post-traumatic stress disorder or traumatic brain injury.

When a parent is depressed or anxious, Murphey said, that stress acts like a contagion and spreads throughout the family.

Infants can become listless, cranky, unable to sleep or refuse to eat. Toddlers can be alternately clingy or withdrawn. They may be sullen or explode with unexpected tantrums. Preschoolers may feel guilty and responsible for a death or for family discord. They may regress, begin wetting the bed or be unable to sleep alone.

Joanna Robertson, who lives near Quantico, Va., said she didn’t understand at first why her then-3-year-old daughter began to act out when her husband, a family doctor in the Navy, was away on the second long deployment in her life. She found her children were “a little naughtier” than normal, “their emotions more raw.”

“I realized that so much of what they experience comes from me. When I was calmer, so were they,” she said. “But when I was sad because my husband was gone, or on edge because there was so much to do and I was having to do it all, they were feeding on that, too.”