"Entrée" used for main course dish in America

By the late 1800s, a typical formal dinner in the UK ran to about six courses: soup, fish, entree, roast (or "joint" - no giggling), maybe another savory course (often a pudding), and dessert. As you'll notice, the entree wasn't the opening act. It was generally a "made" or highly prepared dish - possibly meat and vegetables, maybe sweetbreads or liver - as opposed to the more unadorned roast, but this distinction could be blurry; in the earliest use of entree cited by the Oxford English Dictionary, from 1759, the dish described is a roasted ham. So while one could argue that the entree was the last of the preliminaries, it seems equally defensible to see it as the entrance into a series of what we'd now call main courses. Under main course, in fact, the OED has "one of a number of substantial dishes in a large menu," and in most cases the entree was clearly substantial enough to qualify.

This interpretation prevailed in the U.S., where British conventions held sway, but as American menus became more streamlined in the early 20th century (old-school chefs were already griping about graceless, hurried modern dining as of 1905) some courses got the ax. The roast lost its automatic spot (possibly due in part to WWI meat rationing), the additional savory dish fell away, and soon enough the entree had gone from one of several main dishes to the last main dish standing. 

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Actually, there is indeed a good explanation for why Americans do this.  They got it from the English.  At some point in the 18th century, the English began using entree to mean "a ‘made dish’, served between the fish and the joint".   But in French, entrée  was defined as "qui se servent au commencement du repas" ("serving as the commencement of a meal").  So the English got it wrong first, and that "wrong" meaning simply stuck in America once it arrived here, while the French doubtless harangued the English so much about misuse of the term that the meaning was corrected in the U.K. (well, maybe not, but...).

In America the word eventually came to refer not to the 'made dish' (which was often a ham) but to the main course (usually a ham or some other meat).

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 An entrée (pronounced /ˈɑːntreɪ/ ahn-tray, French "entrance") is a dish served before the main course, or between two principal courses of a meal.[1][2][3]

The disappearance in the early 20th century of a large communal main course such as a roast as a standard part of the meal in the English-speaking world has led to the term being used to describe the main course itself in some areas.[4] This usage is largely confined to North America and use of the term would be considered unusual in British English, even though it is the first meaning given by some British dictionaries[5][6][7] but not others.[8]

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