WASHINGTON — Mark A. Milley never planned to spend his life in the military.

He faced a choice between joining crew-cut cadets at West Point and attending an Ivy League university. As he weighed the decision, his father, a Marine who fought at Iwo Jima in World War II, made clear he did not want his son joining the military, and even enlisted another son to sabotage the young Milley’s West Point visit by getting acquaintances to show him the more miserable parts of the Army college regimen.

Alexander Milley, the father, won the battle — Mark Milley did not go to West Point, but to Princeton, where he grew his hair long and played on the hockey team.

But the elder Milley, who died in 2015, lost the larger war. Something had been planted in his brain, Mark Milley has told friends, that made him believe that he was lucky to be born in America, and duty bound to follow his father’s footsteps. At Princeton, he joined the R.O.T.C.

On Monday, Gen. Mark A. Milley, a four-star Army officer with multiple combat tours during his 39 years of service, will be sworn in as the highest-ranking military officer in the country: chairman of the Joint Chiefs of Staff. He succeeds Gen. Joseph F. Dunford Jr. of the Marines, who was appointed by President Barack Obama.