Today marks the 40th anniversary of the fall of Saigon, when the Viet Cong captured the capital of South Vietnam following a hasty and controversial evacuation by the US.

Though it's been decades since most Vietnam veterans returned home, awareness of the dramatic and harmful effects post-traumatic stress disorder (PTSD) has on American military veterans is relatively recent. The term PTSD first came into use in the years following the end of the war, and it is Vietnam veterans who have thus far taught us the most about what happens when there is a lack of resources and inadequate support for soldiers returning home.

Kara Frame, herself the daughter of a Vietnam veteran, is documenting the effects of PTSD on the lives of three Vietnam veterans — and their families — who served together in the 5th Infantry Regiment in 1968 after being drafted. These photos are part of the project, called I Will Go Back Tonight.

Nightmares are common in those diagnosed with PTSD. Says Specialist Abe Cardenas: "People ask me, 'When were you in Vietnam?' When was I in Vietnam? I was there last night. I was there this morning. Five minutes ago, before you asked me. And I will probably go back tonight."

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Staff Sergeant Tom Frame (the photographer's father) says he fishes as one way to escape his PTSD. It allows him to focus on one thing, preventing his mind from wandering.

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Though his wife and daughter ask him to quit smoking, Specialist Gary Young refuses to. In Vietnam, Young was a medic. His wife, Betty, says that he would only mention his experience there if there was a movie on about the war. Then, he would simply say it "wasn't like that." Young suffered with PTSD for years before a VA psychologist put him under hypnosis to get him to open up. It wasn't that he didn't want to talk, it was that he had suppressed his memories.

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Young holds his youngest grandson, Ethan. Betty says there are few things that help him deal with his PTSD more than spending time with his grandchildren.

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The 1st Battalion (Mechanized), 5th Infantry march north out of Dau Tieng in the summer of 1968. On August 21, they were ambushed and lost 17 men at Ben Cui.

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Cardenas rests his head in his hands as a nutrient-rich formula drips down his feeding tube in a process that can take hours. In addition to PTSD, Cardenas suffered from throat and prostate cancer, both of which he says are related to his exposure to Agent Orange in Vietnam. As a result of the throat cancer, he received a tracheotomy.

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Frame at Fort Bragg, North Carolina in 1967. It wasn't until 1984 that he felt he began healing from PTSD. That was the year he started Chapter 210 of the Vietnam Veterans of America. Being surrounded by veterans again helped him feel comfortable back at home.

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Betty Young would try to stay away from Gary when he was angry or she could tell he was having a "bad day." So she often stayed busy. Having put one grandson to sleep, she spends time on her iPad before leaving to pick the other one up from daycare.

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Abe and ChaCha Cardenas were engaged to be married when his draft notice arrived. Abe wrote to ChaCha constantly while he was deployed, but when he came home, he was suffering from PTSD and alcoholism.

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Gary and Betty Young met after his return from Vietnam. They dated for two months before getting married.

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Vietnamese farmers work in a field as a patrol passes in 1968.

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Frame and his wife, Chris, enjoy a morning with their youngest grandchild, Tommy, who is named after his grandfather. Frame says it's the morning when his thoughts typically drift to comrades who died in Vietnam.

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ChaCha Cardenas had to write Abe a letter informing him that she had given birth to their first child, Martha, while he was in Vietnam. She says she wasn't sure if he would make it home to meet his daughter.

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Chris Frame says it took 15 years of marriage for her to understand the struggle Tom was having with PTSD. Her realization began one day he came home from work crying because he'd had a flashback.

_Follow Kara Frame on Twitter: _@kara_frame