Fallout isn't set in in our hypothetical future, but the future of another timeline—one in which culturally America never left the 1950s and technologically the world went down a path heavy on the vacuum tubes and without color TV. Each new game has added to that alternate history, and sometimes rewritten bits of it, and no doubt Fallout 76 will add and revise even more. The trailer suggests a starting date of October 27, 2102, which is much earlier in the timeline than any of the previous games. It's 59 years before even the original Fallout, and 185 years before Fallout 4.
Below is a timeline of the major events in Fallout's alternate history as it exists right now. While there have been references to events going back as far as the 17th century in the games (like the alien abductions in Fallout 3's Mothership Zeta expansion), the broad sweep of history in the Fallout universe resembles ours right up to the end of World War 2. It's only after 1947, when transistors were popularized in our world but not that of Fallout, that divergence begins, and only in the 1960s that those differences becomes significant.
Captain Carl Bell of the United States Space Administration, piloting the Defiance 7, becomes the first becomes human in space. According to the Museum of Technology in Fallout 3, "this has been constantly refuted by both the Soviet Union and China."
The USA is divided into the Thirteen Commonwealths, each comprising several of the existing states, and changes its flag. The Valiant 11 lunar lander touches down and Captains Richard Wade, Mark Garris, and Michael Hagen of the USSA become the first astronauts to walk on the surface of the moon. In 308 years time, the Valiant 11's dish will be repurposed to increase the distance over which Three Dog can play 'Butcher Pete' at people.
The Sierra Army Depot changes its mission statement, becoming "Responsible for the demilitarization of stores of surplus ammunition". Eventually, it will become home to the Skynet AI as seen in Fallout 2.
The USSA commissions the creation of the Delta IX, the final model of manned moon rocket. Eventually the Delta IX rockets will be repurposed, their crew and instruments replaced by nuclear warheads.
The first Mister Handy Robots are made available to the public.
The final manned moon mission occurs in the same year that worldwide resource shortages begin. Oil prices in the Middle East rise drastically, and the European Commonwealth responds by declaring war. This escalates into a global conflict called the Resource Wars.
The European-Middle Eastern War inspires the US to begin Project Safehouse, which will lead to the creation of the Vaults.
The US increases its military presence in Alaska and establishes the Anchorage Front Line, causing tension with Canada.
The European-Middle Eastern War ends as soon as the Middle East's oil fields run out. Many countries, including the US, devote more effort to nuclear energy research. The European Commonwealth dissolves.
New York's nuclear reactor comes close to a meltdown. Construction of Vault 76 begins.
An energy crisis in China contributes to increased aggression toward the US, who refuse to export oil to them. China invades Alaska.
Chryslus Motors debut the first fusion-powered cars.
The US begins annexing Canada.
US troops are deployed in China. The President walks out of trade negotiations, declaring that US oil won't be for sale or trade.
The US completes its annexing of Canada, and deploys T-51b power armor (made possible by nuclear fusion cells) in China. On July 4, Vault 76 is opened to celebrate the US tricentennial.
On October 23 the Great War begins. As Ron Perlman puts it in the intro to Fallout 2, "The earth was nearly wiped clean of life. A great cleansing, an atomic spark struck by human hands, quickly raged out of control. Spears of nuclear fire rained from the skies. Continents were swallowed in flames and fell beneath the boiling oceans. Humanity was almost extinguished, their spirits becoming part of the background radiation that blanketed the earth." Also on this day, Nuka-Cola Quantum is released to the public.
Survivors of the Great War still living on the surface begin to notice mutation spreading, both among themselves and animals.
The start date of Fallout 76. It's Reclamation Day, again.
In The Hub, a group of merchants seize control of the settlement's water tower and rival merchants unite to fight them. The Great Merchant Wars begin.
Robert House awakens from cryogenesis.
Start date of the original Fallout.
The Vault Dweller, protagonist of Fallout, settles down and founds the settlement of Arroyo with several other exiles from Vault 13 and members of local surface tribes.
The settlements of Shady Sands, Los Angeles, Maxson, the Hub, and Dayglow vote to combine themselves into a federation called the New California Republic.
The ghouls of the Capital Wasteland form the Underworld.
The Boomers settle Nellis Air Force Base.
Start date of Fallout 2.
Caesar establishes a base for the Legion in Flagstaff, Arizona. The Pitt, a raider settlement in what was Pittsburgh, is attacked by the Brotherhood of Steel and almost the entire population is wiped out in an action called 'the Scourge'.
Start date of Fallout 3, or at least its character creation.
The NCR defeats the tribes based around Bullhead City in an action called the Pacification of the Mojave.
The NCR and the Desert Rangers sign the Ranger Unification Treaty.
Galaxy News Radio makes its first broadcast.
Caesar's Legion moves into the Mojave, and the first Battle of Hoover Dam occurs. Boulder City is flattened, but the Legion is defeated when the NCR lure them into a trap. Caesar blames Legate Graham, and he is punished by being set on fire and thrown into the Grand Canyon.
Start date of Fallout: New Vegas.
The Quincy Massacre occurs when the Gunners attack the Commonwealth Minutemen, leaving only 20 of them alive. Start date of Fallout 4.
The furthest event in the timeline of Fallout is a hypothetical one, but if you complete the excellent Old World Blues expansion for Fallout: New Vegas and fully upgrade Blind Diode Jefferson, the AI who has the blues, then the epilogue explains he eventually "created a symphonic counter-frequency that saved Big MT from sonic invasion in 2910. If you didn't hear about it—good."