To insist that a sandwich be eaten with knife and fork is to misunderstand the logic behind the invention of the sandwich in the first place.
While the use of word “sandwich” to describe meat or some other ingredient placed between two or more slices of bread to be conveniently eaten in the hands is attributed to the 18th-century English aristocrat John Montagu, the fourth Earl of Sandwich, who, according to legend, one evening, while playing cards and not inclined towards interrupting the game or soiling his hands in the process of eating requested that his servant bring him a “piece of meat tucked between two slabs of bread,” the concept of conveying food to the mouth held between bread is much more ancient, as evidenced by the traditional cuisines of North Africa and the Middle East which routinely use flat breads, in lieu of forks and spoons, to pick up meats and other ingredients from the plate in order to convey them to the mouth. But if the birthplace of the concept of the sandwich is the ancient world, certainly its international success is due to America—perhaps because of that country’s casual, fast-paced approach to life.
It is unlikely that sandwiches will be served as a course during a formal dinner. But they are oftentimes presented as options on the menus of many formal restaurants—especially those in America or in regions of the world influenced by her. And even in those venues, as formal as they may be, a gentleman is totally in compliance with the laws of etiquette to pick up his sandwich with his hands even though a fork and knife may have been provided—and they invariably will be. But again, common sense should prevail, for one can be “wrong” even though he is within his rights. The purpose of manners is to make others feel comfortable, not to prove social points. So, for example, if the sandwich is exceptionally large, it should first be halved or even quartered with a knife and fork and then eaten in the fingers; likewise, if the sandwich is particularly messy or unstable, it should be eaten in its entirety with a knife and fork. At the end of the day, a gentleman should make sure that his method of eating does not offend or disrupt the comfort of those in his company or immediate vicinity.