
The world doesn’t work better if people help those closest to them. The majority of the strife in the world exists because people help those close to them and treat others as outsiders. You can look at any system ever used and the point of failure is always someone tipping the scales intentionally to favor those near them.
SNW S1E2 - Children of the Comet. This is a tricky one because of potential predestination and the end result being an accidental major break of the prime directive. A warp capable civilization is certain that a comet must destroy a pre warp civilization and refuse to let that change. The issue is that the ship/comet wants the Federation to interfere and does not intend to hit the planet. This is all in Federation space.
Another in Federation space that is also a loose fit is LD S5E7 - Fully Dialated where the crew must recover Data’s head from an alternate reality that has fallen on a pre warp world. The loose fit here is that the only reason they count as external is that it is Purple Data who is not technically a a member of this realities Federation and thus an outsider.
Lastly a decent fit but outside Federation space is Prodigy S1E7 - First Con-Tact. The crew first breaks the prime directive by making first contact with a pre warp civilization but then defends it by stopping a Ferengi from influence the civilization negatively. This is outside Federation space though.