A Quality of Mercy
Summary
While the Enterprise and the USS Cayuga provide supplies to an outpost near the Romulan Neutral Zone, Pike meets one of the future cadets for whom he will sacrifice himself. He decides to send warnings to them to prevent the accident from happening. Pike is then visited by an older version of himself from a future where the accident does not happen. He reveals the consequences of this decision: seven years in the future, the Enterprise sees a Romulan Bird of Prey starship destroy the outpost. They track it with the USS Farragut, commanded by Captain James T. Kirk who suggests an aggressive approach. Pike insists on negotiating and the Romulan commander agrees to a ceasefire, but his sub-commander disagrees and summons an armada of Romulan warships. The Enterprise is damaged, Spock is severely injured, and the Romulans declare war on the Federation.
Summary from Wikipedia, CC BY-SA 4.0.
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