For Dr. Tom McDonald, the journey from his home in Ireland to an internship at Mayo Clinic in the mid-1960s included a year-long detour to Vietnam.

That’s just the shorthand version of how a physician who was not yet an American citizen ended up on the front lines of a war that most American men were anxious to avoid.

McDonald came to the U.S. in 1964 after graduating in 1963 from the Royal College of Surgeons in Dublin. His visa was initially good for one year but could be extended.

"There was one condition," McDonald said. If he intended to stay beyond that year, "You had to sign on with the Selective Service. I was beginning to feel that this (America) was the place for me. So I signed on."

He was promptly drafted into the U.S. Army. Then, thinking it might be an adventure, McDonald volunteered to go to Vietnam, where an American military buildup was in full swing.

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"It seemed exciting," said McDonald, a retired Mayo Clinic ear, nose, throat doctor. "I had always been interested in matters of the military."

So, after rudimentary military training, McDonald found himself in September 1966 stepping off a plane into the hot, humid air of Saigon — a non-American citizen dropped into the middle of a war that threatened to fracture his adopted country.

"My first thought was excitement and fear at the same time," McDonald recalled. "I thought, ‘Why is everybody wearing weapons? Boy, this is serious stuff.’"

It became even more serious when McDonald was assigned to the 12th Evac Hospital at Ku Chi, west of Saigon. There, in between operating room shifts, he slept with a loaded .45-caliber handgun under his pillow.

In his unpublished memoirs, McDonald writes of the daily routine, the hardships, the comradeship, the lessons he learned, the pain of watching young, healthy men torn apart by bullets, mines and mortars.

He rarely had time to think about the political implications of the war, he said. "You were so busy," he said. "A big helicopter would come in with 30 casualties." The hospital itself was frequently attacked with mortar fire. "That was very frightening," he said.

Through it all, McDonald said, he and his fellow doctors and nurses took pride in treating everyone. If the five most seriously injured men were enemy soldiers, he wrote in his memoirs, they would be tended to first. "We acted as doctors, not as politicians," he said.

When his year-long tour was up, McDonald returned stateside, where his wife, Mary, was waiting for him. On the plane home, he said, "I thought, ‘I made it through this, I’m going home.’"

Home, though, was a new country. He had made up his mind to stay in America and become a citizen. "I was proud of myself for having fulfilled my obligation to this country," McDonald, now 77, recalled.

He was discharged from the Army in March 1968, and started a residency in the ENT department at Mayo Clinic. In 1972, he became a consultant, and later served as vice present of the clinic’s board of governors. He retired in 2007, but continues to attend weekly education conferences at the clinic.

As for the incongruity of an Irish immigrant at a U.S. Army field hospital in Vietnam, McDonald said, "I think it’s testimony to the American spirit. They’ve always welcomed strangers. This country was built on people coming here from other countries."