Before we get to the awesome baseball episode that is "Hit The Diamond", Lapis and Peridot have some serious relationship building to do in "Barn Mates". Although they used to be part of the same team, it's clear that there was a lot of friction between Lapis and Peridot, and this all comes pouring out in a really nice way in "Barn Mates". We also got to see how far Peridot has come, from a quipping and manic villain to a would-be hero (who's still pretty quippy and manic, to be fair).
After the intense plot building of the first two episodes of Season 3, "Same Old World" and "Barn Mates" take a far slower, character-building pace. But they're not without their plot hints — "Same Old World" gave us an intriguing flashback to the Gem War, in which Lapis could have been a war reporter or a spy.
"Barn Mates" mostly focuses on Peridot proving herself to Lapis (in a totally platonic and definitely not romantic way... ahem), and then Yellow Diamond's "roaming eye" probe shows up.
It All Comes Back To The Gem War
The roaming eye seems pretty powerful, though we don't get to see whether it has firepower, and we have to wonder why Yellow Diamond didn't send more of these before to take out the Crystal Gems. Of course, in the final moments of the episode we discover that actually, the probe was piloted by a Ruby (and there are probably more inside).
It's very clear that Homeworld have a massive interstellar fleet at their disposal. We know that they have an empire, and their forces are probably unmatched in the galaxy, from their method of using entire planets as breeding grounds for new gems. How then, did Homeworld lose the war against the Crystal Gems?
In "Same Old World", Lapis said that she was "confused for a Crystal Gem", which is interesting as this small statement makes it clear that Rose Quartz had an army, and they were all known as the Crystal Gems. We knew there were other rebels from the episode "The Answer", but with this offhand comment Lapis has confirmed something — there were other Crystal Gems but only Rose, Garnet, and Pearl survived.
So what happened to the other Crystal Gems? Lapis also mentioned that she was interrogated and trapped, implying that other captured rebels underwent a similar treatment. Are there any surviving Crystal Gems left on Homeworld? The questions never end.
Earth Just Isn't That Important
Leaving those stray questions aside, the roaming eye's presence in "Barn Mates" makes it clear, again, that Yellow Diamond has the power to attack Earth any time she wants. Obviously, she assumes that the Cluster will just destroy Earth once it comes to fruition, but all of this has an interesting implication for the Gem War — the Crystal Gems didn't really win at all.
It's never been explicitly spelled out, but we can assume that Homeworld's forces retreated from Earth not because Rose's rebellion won, but because they had implanted the Cluster. This does mean that the Crystal Gems posed enough of a threat for Homeworld to resort to such a method, and they couldn't just be outright defeated. But with most of Rose's soldiers gone, and the Cluster at the Earth's core, it's starting to look like Homeworld had more of a victory than we realised.
Of course, it's entirely possible that Earth just isn't that important in Homeworld's vast empire. Or at least, it wasn't until Rose started fighting for it.
Homeworld was willing to abandon their colony and use the Earth for the Cluster when it looked like they would lose otherwise. But with Rose's mission to protect humanity being continued by Steven, the Crystal Gems, and now Lapis and Peridot, Homeworld might be forced to view Earth as more of a threat. And that's when the problems will really begin for our heroes, when Yellow Diamond decides to take matters into her own hands.