Earth's 3190 decision to rejoin the Federation it had helped found, after the Dark Matter Anomaly crisis was resolved.
After the Burn, Earth left the Federation and turned inward, defending its own borders and keeping outsiders at a distance. The crew of the USS Discovery had already met the isolated United Earth on their arrival in the 32nd century.
In 3190 Species 10-C shut down the Dark Matter Anomaly that had been heading for Earth, then used a wormhole to send Discovery home. Earth decided to rejoin the Federation, returning the founding world of the alliance to its ranks.