I think that all ships before the Defiant were built wholly or, in the case of the Constitution class and all similar ships, partly as exploratory vessels. The mandate "to explore strange new worlds etc." seems to be a large part of Starfleet's purpose (it's not a purely military institution). Defiant was simply the first to be designed without that mission in mind. It has a small crew and few if any of them are dedicated to science and research, unlike all other classes of ship until then.Dragon1 said:DS9 goes as far to say that the Defiant was the first true 'warship' built by Starfleet. Interesting then how the Enterprise during the movie era was known as a Constitution-class Heavy Cruiser. Also, The Motion Picture referenced a 'Dreadnought Entente'. Heavy Cruiser and Dreadnought have a warship connotation to me.