The Advent of World War III

Okay so this doesn’t actually happen in 2024, but it’s important to note after last couple of events—turning points in human history for the better, without which a United Earth and the concept of Starfleet and eventually the Federation wouldn’t have existed—that things don’t immediately start improving. In fact, they get much, much worse: World War III breaks out two years later in 2026.
Continuing global unrest sees the United States enter thermonuclear war with a pan-nation alliance known as the Eastern Coalition. Lasting into the early 2050s, World War III sees the death of six hundred million million people, the eradication of almost every major city on Earth, and the complete destruction or dissolution of many national governments. Society as most of Earth had known it for hundreds of years collapsed entirely, and if not for the arrival of a group of Vulcans in 2063 to kickstart humanity’s ascent to the interstellar sphere, it likely would’ve stayed that way as the remnants of the species navigated the fallout of the post-apocalypse.