Morale, discipline, esprit de corps although not always close synonyms, are comparable when they mean a condition or spirit which holds together a body of persons.
Morale usually applies to the qualities of an entire body of men (as an army or a regiment, a people, or a community) with respect especially to their courage and endurance under stress, but it sometimes may refer to an individual in his capacity of a member of a body held together by such qualities.
Discipline applies to the order maintained and observed by a body or an individual that is or has been subjected to training (as in uniform behavior, in control over the passions or other individualistic traits, or in military exercises) so that the whole moves under command as one and the individual thinks of himself only in relation to others.
Esprit de corps especially applies to the spirit of loyalty that is manifest in a body (as a profession or a society) by jealous regard for the honor or the interests of the body as a whole or for fellow members as belonging to it; often esprit de corps implies a spirit that distinguishes one body and brings it into opposition to others.