Like a wet dream from the first season, "United" continues the Federation Trilogy by eventually pitting Archer and Shran in "Amok Time" combat. (It should be noted that like the Vulcan Trilogy earlier in the season, I'm refraining from calling any individual installment from it an outright classic, though in this instance, "United" would be that episode.)
Basically all the stuff that happened in "Babel One" degenerates. Reed and Trip (their last real adventure together, making the episode all the more historic) are trapped by the Romulans, who are more actively involved in this installment and the Andorians and Tellarites are still getting along miserably. And yet Archer is really starting to figure out what it takes to get past these problems. He started out as the guy who happened to be the son of the warp drive innovator who made it possible for Starfleet's expanded mission to happen, and was a captain by virtue of his enthusiasm, not so much because he was a diplomat. Yet being quintessentially human was always seen as a virtue in Star Trek (see: James T. Kirk), and somehow Archer figures out how to make enemies see clearly. It's a step in the right direction.
It's an episode that needed to happen, and does, and as part of a larger story, and that probably needed to happen, too. Ideally, there would have been a later season filled with this kind of stuff. Imagine how much more we could have learned about the Federation!
(Plug for a new book.)
Anyway, watch this one if none of the rest of the surrounding story.
franchise * series * essential * character
Memory Alpha summary.