So I was at Prydin and got the message that Sanctuary was under attack. I am on the other side of the country, and my village had good security , so I went forward with the mission to attack the mutants at the fort. Turns out after the battle I got the message thatI failed defending Sanctuary. I checked my pip boy: population fell from twelve to five, no food production, defense down from 49 to 20. What now? Should I reload, go back there before going to the Fort? I don't like failing, and I failed as a leader to my community. Or is this an event that has little repercution? Ouch...

Not just little repercussions, closer to zero. Just replant, rebuild, and rearm and you'll be fine. If you build it, they will come

Would the attack still happen if you reloaded, I wonder? Not sure what mechanism governs attacks triggering.