Warning! Spoilers for Star Wars #18 ahead!
In Marvel's most recent issue of Star Wars, it's confirmed that Han Solo's rescue was crucial for the Rebels' future success against the Empire, including the destruction of the second Death Star. This is largely due to the Rebellion's leader Leia Organa and how she felt in the aftermath of Empire Strikes Back. With the fleet scattered and being hunted by the Empire after the Battle of Hoth, the future of the Rebellion is looking grim. Furthermore, this new issue confirms that Leia's inability to save Han prior to Return of the Jedi has led to her abandoning all hope, failing to be the leader the Rebels need.
Previous issues of Marvel's Star Wars from writer Charles Soule and artist Ramon Rosanas have seen Leia essentially leading the Rebellion on auto-pilot. Having lost Han on Cloud City to the Empire and Boba Fett, Organa has been determined to find a way to save the man she loves. As a result, when Fett loses the frozen carbonite body of Solo to Crimson Dawn at the start of the War of the Bounty Hunters, Leia sees an opportunity to save Han. However, despite being seconds away from freeing and rescuing Han, she and the Rebels ultimately fail by the event's end.
Now, Star Wars #18 reveals the aftermath of the War of the Bounty Hunters, featuring Leia in distress believing that Han is dead (when in fact he has been recovered by Fett and taken to Jabba the Hutt). Grieving with Han's first mate and best friend Chewbacca Leia's outlook is incredibly bleak. It's also an indication that without Han Solo's eventual rescue as seen in Return of the Jedi, she might have never led the Rebellion to victory against the second Death Star, the Rebellion's definitive victory that paved the way for the creation of the New Republic.
Believing Han to be dead, Leia has zero hope. Furthermore, she also confirms what's become evident in the wake of Empire Strikes Back: she's been going through the motions like a programmed droid rather than the emboldened and hope-filled leader the Rebellion actually needs. The meeting with Crimson Dawn's Lady Qi'ra is bound to be quite the dynamic conversation, seeing as how Qi'ra was the woman Solo loved before Leia.
The loss of Han Solo was extremely detrimental to the Rebellion based upon how it affected Leia.Without Han Solo to give Leia Organa and the rest of the Rebellion hope, it's possible that they could have failed to destroy the second Death Star, which turned the tide of the Galactic Civil War in their favor at the end of the original trilogy of Star Wars film.