Buck Rogers in the 25th Century Wiki

The United States undergoes a devastating nuclear attack, and all major U.S. cities are destroyed. The U.S. counterattacks, and a second wave of missiles is launched by enemy forces. This war is no doubt waged primarily between NATO forces and those of the Warsaw Pact.

In the minutes before what he knows will be the final destruction not only of America, but ultimately of the Earth, Major Jim Petersen, USAF, records a videotape at NORAD headquarters in Colorado. As a result of coming into possession of surveillance tapes from Air Force intelligence, Petersen is under the mistaken impression that Buck Rogers was part of a conspiracy within the American armed forces to obtain the secret nuclear launch codes, and initiate a pre-emptive strike against the enemy without civilian authorization. Prior to this discovery, Petersen had considered Buck to be his best friend. Completing the recording, he places it in a fireproof box. The box is later recovered by the Earth Federation in 2493. ("Testimony of a Traitor")

The second wave of missiles from enemy powers strikes, destroying what remains of the United States.

A general nuclear war rages throughout the Earth killing millions.

Radiation and disease in the aftermath of the nuclear war kills many millions more. Civilization begins to degenerate. The collapse happens very quickly, and bodies are hurriedly buried often without grave markers.

According to Dr. Goodfellow, this destruction may have occurred because there was no Guardian in place at this time, who could have protected the Earth. ("The Guardians")