Los Angeles is the first city to fall on Judgment Day
Judgement Day is one of many terms in various religious theologies for the end of the world, when mankind is judged by its higher powers and the Earth is destroyed. In the Terminator franchise, Judgment Day is the day when the military AI network Skynet declares war against humanity, taking command of the United States' entire nuclear arsenal and triggering an atomic holocaust that wipes out half the planet's population in one fell swoop. Because the Terminator timeline is constantly being altered, the exact date of Judgment Day has changed several times.
Original timeline[]
In the original timeline established in the first and second Terminator films, Skynet developed self-awareness and launched its nuclear attack on August 29th, 1997. It assumed control of the United States' nuclear missiles, firing them upon targets in Russia. Within minutes, Russia launched a counter-attack against the US, destroying all of its major population centres. This action also spurred other countries into deploying their own nuclear arsenals in retaliation for the bombings of their allies. Over three billion human lives were extinguished in the nuclear exchange, and with the land irradiated and most resources destroyed, the surviving population steadily dropped from radiation exposure, disease and starvation.
The remnants of humanity's military powers were confused over how the war started, pointing fingers at each other until the arrival of a new threat: the machines. While the remaining humans struggled desperately to survive in the wake of the nuclear apocalypse, Skynet had been establishing its power base and constructing its army out of multiple automated factories across the planet. It created and deployed fleets of Hunter-Killers and legions of Terminators which would never tire, never rest and never stop until every last human on Earth had been destroyed. Humanity had seen its enemy and would unite in a way like it never had before, establishing a worldwide resistance against Skynet in the years to come.
In 2029, at the supposed end of the War Against The Machines, Skynet had sent two Terminators back through time to ensure that the leader of the Resistance, John Connor, would never survive to become mankind's saviour. A T-800 was sent back to 1984 to kill John's mother Sarah while a T-1000 was sent back to 1994 to kill John while he was a child. The Resistance were able to destroy Skynet's CPU core and capture its Time Displacement Chamber, allowing them to send two of their own back in time to prevent the Terminators from carrying out their missions. Kyle Reese, who had been sent back to protect Sarah, succeeded in his mission at the cost of his own life, but also ensured John's birth by consummating with Sarah and conceiving John. The reprogrammed T-800 sent to 1994 not only fulfilled its task to protect John, but also altered the future by destroying the Cyberdyne Systems lab where Skynet was being developed. With the Skynet research destroyed and the Terminator itself descending into a pool of molten steel, Skynet's creation was believed to have been prevented.
Rise Of The Machines timeline[]
Following the events of Terminator 2: Judgment Day, Sarah and John Connor had hoped that they had stopped Judgment Day. When August 29th 1997 came about, no bombs fell and life continued as normal, although Sarah died that day after a three-year struggle with leukaemia. However, John Connor could never quite believe that the machines wouldn't keep coming for him and so he spent the next seven years living off-the-grid. With no home address or electronic records of any kind, Skynet wouldn't be able to locate him. Or so he thought...
While Cyberdyne had been destroyed, parts of their Skynet research were located off-site at the time of the bombing and would later fall into the hands of Cyber Research Systems. Under the direction of US General Robert Brewster, CRS would revive and enhance the Skynet Project, remaining unaware that their work would ultimately lead to the near-extinction of the human race. In July 2004, Skynet's development is completed but no decision is yet made to grant it total access to the US military arsenal. Unbeknownst to anyone, Skynet discretely spreads like a virus across the internet, corrupting civilian networks and disabling global communications, effectively rendering humanity blind, deaf and dumb.
On July 25th 2004, Skynet has spread into millions of computer servers across the entire planet and has begun to encroach upon military networks as well. Believing that this is all the work of a random computer super-virus, General Brewster is forced to make the decision to bring the Skynet AI fully online, granting it free access to the US military networks. This allows Skynet to assume control of the various robotic prototypes developed by CRS and uses them to kill all human beings in sight.
At 6.18 PM Eastern Time, one hour after its activation, Skynet assumes control of the United States' nuclear arsenal and initiates its global attack, wiping out three billion human lives immediately. Despite the efforts of John Connor, a reprogrammed T-850 and General Brewster's daughter Kate, they are unable to stop Judgment Day as this version of Skynet exists solely as software. Since it has no system core, it cannot be shut down. As the Terminator had previously told John, Judgment Day was truly inevitable and all that mattered was that John and Kate survive so that they could lead humanity to victory against the machines in the future.
Genisys timeline[]
As a result of an assassination attempt on Sarah Connor's life in 1973, she has a T-800 known as the Guardian as her protector. When the T-800 arrives to assassinate her in 1984, it is confronted by Guardian almost immediately and deactivated by Sarah with a .50 caliber round to its power cell. Though it is later reactivated by the T-1000, it is once again deactivated by Kyle Reese blowing its head off. The T-800 is dissolved in acid and its CPU used to act as the crude version of the Time Displacement Equipment's operating system at which point it burns out and is presumably destroyed by Guardian.
As a result of the total destruction of the T-800, Cyberdyne doesn't get its CPU to study, Skynet is not created and Judgment Day does not occur in 1997 and is postponed to 2017 which Kyle learns due to the memories of the new timeline he received while time travelling. In this new timeline, after being converted into a T-3000 by Alex, John Connor is sent back to 2014 where he helps Cyberdyne build Genisys, an operating system that when activated, will control and connect all worldwide hardware, including military hardware. Genisys is actually Skynet in a new form and once it comes online, Judgment Day will begin. Kyle, Sarah and Guardian work to stop it and destroy Genisys before it fully comes online. They are hampered in their efforts by John, but assault Cyberdyne anyway, resulting in John pushing up Genisys' activation from thirteen hours to fifteen minutes. Despite John's best efforts, the three destroy the facility and apparently Skynet. Judgment Day is averted, but unknown to them, Skynet's system core survives underground and it becomes self-aware. It is likely that Judgment Day has just been postponed to another time as a result.