World War III was fought using nuclear weapons
World War III was a period of conflict on Earth that would eventually escalate into a nuclear cataclysm and genocidal war over issues that included genetic manipulation and human genome enhancement. World War III itself ultimately lasted from 2026 through 2053, and would result in the death of some 600 million Humans.
Among the parties involved was the Eastern Coalition (ECON) whose direct attacks included those against the United States. One of the key players in the Third World War was Colonel Phillip Green who led a violent faction of ecoterrorists during the war. He and his troops personally killed hundreds of thousands of individuals affected with radiation damage and other "impurities," using as a rationale that it was necessary in order to prevent their passing on such traits to later generations. Their actions resulted in the deaths of over 37 million people. The war additionally influenced the future powerful xenophobic movement of the mid-22nd Century on Earth known as Terra Prime.
The war finally culminated in 2053, when several of Earth's governments met in San Francisco to declare a cease-fire, effectively ending the War. By that time, many of the planet's major cities and governments were destroyed, and 600 million people had been killed.
First Contact with the Vulcans in 2063 lead to increased recovery from the war for parts of the world. However, despite this contact, various parts of Earth were still affected by what became known as the "post-atomic horror." In 2079, for instance, one such culture (perhaps in Asia) reverted to a state of near-barbarism that followed the credo: "Kill all the lawyers" and "Guilty until proven innocent."
Humanity eventually pulled itself together. Fifty years after First Contact in 2063, war, poverty, and disease had been eliminated from Earth. Within 100 years of the first warp flight, humanity had solved most of the problems that had plagued them for centuries, and worked to become a full member of the interstellar community.