Although this often comes as a surprise to people who haven’t read the manga, Sailor Venus is technically the leader of the Inner Senshi, not Sailor Moon. And Sailor Mars is her second-in-command (a concept that’s heavily expanded on in PGSM and, to a lesser extent, the anime). Venus’ leadership is mentioned numerous times in the manga:
Basically, Venus is the true leader because Sailor Moon was never really supposed to exist in the first place. She’s the Princess, yes, but Princesses are meant to be protected; warrior princesses were/are a fairly radical concept because, for the most part, royalty is sheltered and kept out of harm’s way. Princess Serenity was not a warrior in past, nor was she ever meant to be a warrior in the present or the future. In the manga, Luna admits that she had only trained Usagi as a Sailor Soldier in order to protect and hide her from the enemy.
Anyway, during the Silver Millennium, before Sailor Moon existed, Venus lead the Guardian Senshi in the effort to protect Princess Serenity. In the few flashbacks we see to the Silver Millennium, Venus was almost always at Princess Serenity’s side, protecting her and instructing the other Senshi on what to do.
After they were reborn, Minako was the first to awaken; after all, as the leader, it’s her responsibility to take care of the others and have the experience necessary to deal with any threat. Because of this, she awoke a full year before the rest of the Inners (at age 13) and trained as Sailor V. After she had enough experience, she regained her memories of her past life and became Sailor Venus; soon after, she and Artemis began to watch over the other girls.
Meanwhile, the reborn Princess, Usagi Tsukino, became Sailor Moon assumed the leadership role. Later on, Venus herself (as the most experienced Senshi) continues to act as a leader by forming strategies and taking charge whenever Sailor Moon is missing, incapacitated, or otherwise incapable of being the leader herself. This is especially evident in the first and second arcs, where Sailor Moon was still very inexperienced. As time went on, Sailor Moon matured and became a much more capable leader, although she still relies on Venus occasionally.
A huge symbol of Venus’ leadership is the Holy Sword. She wielded it during the Silver Millennium, using it to protect the Princess; she even killed Beryl with it.
Countless years later, when the Senshi visit the ruins of the Moon, they come across the Holy Sword, stuck inside a pedestal. Mercury and Mars try to pull it out but fail; even Jupiter’s Herculean strength doesn’t make it budge.
However, when Venus tries, the sword is freed with no resistance whatsoever.
This directly mirrors the myth of King Arthur and the Sword in the Stone; After the King of England died, his advisor, Merlin the sorcerer, enchanted his sword and placed it in a stone, announcing that only the King’s true heir would be able to remove it. Many tried to remove the stone, but they all failed. It was only when Arthur, the King’s illegitimate son, took hold of the sword did it finally remove itself from the stone, signifying that he was the rightful heir to the throne. Likewise, the Holy Sword is only freed when Venus, the true leader of the Senshi, tries to remove it.