Smallville had one of the only live-action versions of the Justice League, but how did the series bring the group to life? The Superman prequel series spent its early seasons focusing just on characters from the world of Clark Kent, but after a while, the series began to explore heroes and villains from other corners of the DC Universe that weren't directly connected to Superman, Metropolis, or Smallville.
Smallville started laying the groundwork for the Justice League - intentionally or unintentionally - as early as season 4 with the introduction of Bart Allen aka Impulse (Kyle Gallner), the first person who could outrun Clark (Tom Welling). Bart is the first DC Comics superhero other than Clark to appear on Smallville. Bart drops a hint about the Justice League when he suggests that the two of them form a "league." In season 5, Clark encounters Aquaman (Alan Ritchson) and Cyborg (Lee Thompson Young) in two separate episodes and enjoyed team-ups with both of them. The sixth season adds Green Arrow (Justin Hartley) to the series as a recurring character who tries to make Clark realize that there's a whole world out there that needs saving.
After Green Arrow's debut, all of the pieces are in place. In Smallville season 6, episode 11, which is appropriately titled "Justice", Oliver secretly puts together a plan to take down Lex Luthor's 33.1 facility, a place where Lex (Michael Rosenbaum) experiments on people who are meteor-infected. It's revealed that Green Arrow recruited Cyborg, Impulse, and Aquaman to attack 33.1, but Clark wasn't invited. After Impulse is captured, Clark breaks into 33.1 himself, but is thwarted by kryptonite. Once Aquaman, Cyborg, and Green Arrow arrive, the five heroes team up.
With Clark's help, the team succeeds in blowing up 33.1. The scene culminates in an epic moment where all five heroes walk away from the explosion. It's the first time where Clark works alongside more than one super-powered hero, and it's a special moment for the show and for the character, as it gave Clark a chance to work as a member of a team. At the end the episode, Oliver and the others leave to do good in other places in the world, and invite Clark to go with them, but Clark elects to stay in Smallville to deal with a different problem.
Smallville deliberately dances around calling Oliver's new team the "Justice League". As if wasn't already obvious this was Smallville's take on DC Comics' biggest superhero team, Oliver suggests the team's name include the word "justice." The Justice League is featured numerous times for the remainder series, and each member returns at one point or another. Several other heroes are made members as well, including Black Canary, Martian Manhunter, and more, but none of the later Justice League team-ups equal the scale of "Justice". This is far from the last appearance of the Justice League on Smallville, but its arguably the most memorable, as it's the first and only time the five of them appear on screen together.