Green Arrow (Justin Hartley) is the hero who put together Smallville’s Justice League, but everything he built was taken away by Clark Kent (Tom Welling) in season 8. Green Arrow’s band of super-powered vigilantes was the TV show’s take on DC’s most iconic superhero team. Smallville deliberately avoided using its full name, but used enough references to make it clear who they were.
Starting in season 4, Smallville introduced several DC Comics superheroes and in season 6, brought them together for an epic team-up. It was revealed that Oliver Queen, who was committed to putting a stop to the illegal operations conducted by Lex Luthor (Michael Rosenbaum), secretly recruited three heroes behind Clark’s back. Under Green Arrow’s leadership, Bart Allen (Kyle Gallner), Aquaman (Alan Ritchson), and Cyborg (Lee Thompson Young) worked with Clark to destroy one of Lex’s facilities. In the aftermath, Smallville's Justice League was formed. Clark opted not to be a member at first, but ended up serving with them on multiple occasions. As time ed, more Smallville heroes ed their ranks.
Tensions developed between Clark and Oliver over their moral differences in season 8, and they escalated even further when Oliver killed Lex Luthor. In the season 8 finale, Clark was in the barn making a plan to beat Doomsday with Bart and Black Canary (Alaina Huffman) when Oliver arrived. Oliver argued that Doomsday’s human alter ego, Davis Bloome (Sam Witwer), should be killed. That’s when Clark angrily told Bart and Dinah the truth about Oliver’s decision to kill Lex. Clark then said to Oliver, “you’re not one of us anymore”, effectively cutting Oliver’s ties to the Justice League (even though Bart and Dinah ended up going against Clark’s decision and listened to Oliver anyway for this particular mission).
From then on, Clark began to fill a leadership role with the Justice League, giving orders and making inspirational speeches in true Superman fashion. Clark did decide to forgive Oliver when he realized he had made a costly mistake by trying to save Davis, but despite that, Oliver still became disillusioned with his life as the Green Arrow and it took some time for him to go back to being a hero again. But by this time in season 9, Clark had fully grown into the position of leader, with each member of the Justice League now fully behind Clark and ready to follow his commands.
The team that Green Arrow worked tirelessly to put together ultimately became Clark’s. It was unfortunate that all the effort Oliver put into forming the Justice League had such a poor result for him, but what his takeover of the Justice League boiled down to was Clark’s strict no-killing rule (which he expected others to follow) and his evolution as a hero. Back in Smallville season 6, Clark was barely even ready to be a Justice League member, let alone the group’s leader. But as more conflicts arose and Clark developed a greater sense of responsibility, his commitment to being a hero and his confidence in his decisions grew.