THE U.S. MARINE CORPS underwent what was probably its most traumatic post-Vietnam experience when it was sent on a strange and ill-defined mission of maintaining a "presence" in Lebanon starting in the fall of 1982.

It was a mission for which it had no previous training and for which it was ill-prepared and ill-served by American politicians, and this in itself makes the first book dedicated to the Marine view of that experience of particular interest.

Eric Hammel's The Root, the Marines' nickname for Beirut, is a detailed narrative of the life and death of the U.S. Marines during the roughly 18 months they were stationed at Beirut International Airport next to the teeming Shiite and Palestinian slums on the southern outskirts of the capital.

It is original in that it is based on 200 in- depth interviews with the participants made possible by the Marine Corps, which gave Hammel unrestricted access to the officers and foot soldiers who served there and survived the horrendous truck bomb explosion in October 1983 that killed 241 of them.

Hammel has put together an almost day- by-day -- sometimes hour-by-hour -- account of the evolution of their stay in Lebanon from the start of a generally friendly welcome there through long months of boredom until the slow slip into hostilities with the nearby Shiite community that culminated in "the bombing."

Specialists in Marine Corps history, buffs of Lebanon's torturous contemporary history and others with a very special interest in the history of U.S. foreign policy will find Hammel's work of particular interest because it presents the first detailed "foxhole viewpoint" of the whole episode.

Others will probably find Hammel's account too narrow in scope and swamped in the specialized lingo of Marine Corps acronyms ( MAU, MARG, MSSG, BLT and the like) to want to plow through it.

For all its value as an initial Marine history of that gruesome experience, The Root devotes scant space to the larger issues of the policy-making that brought the Marines to Beirut and kept them there on what became an absurd mission as they turned into sitting ducks for Druze artillery in the hills overlooking their poorly defended bunkers.

Even the fascinating issue of how the Marine mission of establishing a "presence" came to be defined, by whom and where, is not addressed, though Hammel remarks early on it was a phase "no Marine commander had ever before seen on an operations order."

Similarly, the author notes how the Marines unwittingly became bogged downed in the quagmire of Lebanon's bloody internal politics as the United States turned to arming and training the Lebanese army. How the Marine command saw and felt about this, whether it realized the implications of its own and the U.S. government's actions, are issues beyond the scope of the book.

In fact, a host of questions intimately linked to the Marine mission there is not addressed, leaving the book for all its merits as a diary of the Marine mission without a broader context in which to judge its evolution and final humiliation.

Even the issue of how the Marines felt about their mission is scarcely touched. On the very last page, Hammel notes that there was no consensus among his 200 interviewees on this question. Asked what they thought they had accomplished, about half, he says, replied, "We bought the Lebanese more time," while the other half replied "nothing."

In his only judgment on the whole episode, Hammel's last words are, "As time passes, 'nothing' looks more and more like the correct assessment."