The answer Mr. Trager gives to the question in his title is not at all an original one--in fact it is the one the present administration in Washington has been reiterating constantly--but he has been able to argue it rather more firmly and persuasively than most of its other defenders. We must guarantee the independence of South Vietnam, he says, because we have promised to do so and because it is in our own best interest to do so. We must stop the Communists of North Vietnam from invading their neighbors. We must prove to China that ""wars of liberation"" will not work. Neither we nor South Vietnam are bound by the Geneva Agreements, and in any case, the North has broken them on numerous occasions, too. We do not necessarily have to approve of Ky's government to defend it by whatever means are required. The struggle is not purely a civil war, it is also a ""war by invasion""--and, says Mr. Trager gratefully, ""at long last, a President of the United States has so named it."" Therefore we are justified in bombing North Vietnam. The arguments to the contrary on each of these points are not quite dismissed out of hand, but they are hardly represented here in their most favorable light. The closing words are from President Johnson's Johns Hopkins speech of April 7.