#if you still prefer to think that the barrier was there pre-tree-Thalia that’s totally fair

[whydoesthiskeepappening]

Ok, so I went back and found the scene where Grover talks about Thalia’s death in The Lightning Thief. His exact worlds are, “Her spirit still helps protect the borders of the valley. That’s the hill is called Half-Blood Hill.” So, I can see how that can be interpreted as there having been a border before, that’s a completely valid take from this explanation, and since you said you read a translation it’s entirely possible that the way it was translated implied that there was already a barrier even more than the original.

That being said, earlier in the book, when Percy, his mom, and Grover are running from the Minotaur, Sally points out the Thalia’s tree as the “property line” and says that she can’t cross it, making Thalia’s tree a marker of where the boarder actually is. If there was already a barrier there when Thalia, Annabeth, Luke, and Grover made it that far then Thalia wouldn’t have had to stay behind, the barrier would’ve given them enough time to at least get help from Chiron and other campers and it most certainly would’ve sounded like a better option than leaving one injured person behind to fight off an entire army of monsters.

As for how the camp lasted that long without a magical barrier, Grover also explains that children of the Big Three have a stronger aura than most half-bloods that attracts monsters (I think it’s later implied that while monsters can smell all demigods that’s only when they’re either nearby or right in front of them, whereas monsters are actively “pulled” towards extremely powerful demigods like child of the Big Three) and since there weren’t any children of the Big Three at camp they probably weren’t being attacked by any extra danger monsters frequently enough to completely overwhelm them.

Here are pictures of the scene from the book:

I feel like a lot of people forget that the barrier around Camp Half-Blood didn’t exist until Thalia died. People also forget that the hill is called “Half-Blood Hill” because Thalia died there. Not to mention that it was Thalia’s spirit protecting the camp, I don’t know how that worked after Thalia got separated from the tree, but it would’ve been cool to see her be able to like, boost the strength of the barrier or something