Star Trek: Everything We NOW Know About The Burn

Discovery's nacelles had barely cooled down before the first shocking revelation, post-Burn, was uncovered. Travelling to Earth, the ship was intercepted by a militaristic force who were none-too-friendly. These were the defenders of Earth, and Earth alone. After the Burn, the planet that had housed the seat of government for the United Federation of Planets for almost a millennium, seceded from that organisation.

As many of the ships destroyed in the Burn were Starfleet vessels, the Federation feared, before they understood the cause of the event, that Starfleet would present a tempting target for whoever was responsible. They relocated their base of operations away from the planet, heading out into the galaxy aboard a space station. Feeling exposed and abandoned, Earth closed its borders. 

By the time Discovery arrived in orbit, the United Earth Defence Forces had replaced Starfleet, and Earth was engaged in an ongoing conflict with raiders who turned out to be from Titan, Saturn's Moon. They, too, were humans, though a century of separation had driven further wedges between them. 

Starfleet diplomacy helped to settle that dispute, though Earth wouldn't rejoin the Federation for some time, citing its reliance on its independence and mistrust of the organisation. While it would eventually take its place in the Federation again, it would not return to the centre of the alliance. That honour was granted to another early member.