The "Digimon Tamers" series is very much a case of "take it as it comes." Everything that happens follows on from something that's happened before, in a nice, neat, orderly line that has no holes that need filling - there's really no need for a timeline for it. However, there are a great many things that happened in the past, to make the Tamers world what it is. And here is where you can read about them.
Note that any text in BLUE is factual, and actually happened in the real world. Or at least, in the case of things like SIGINT and Project Echelon, *allegedly* happened, as the government denies their existence.
1939-42
The Atanasoff-Berry Computer (ABC) - the
very first computer - is built, and lays down the
foundations of the Digital World (from Brave Tamer video
game, see next entry).
1943-1946
The first patented digital computer, ENIAC
(Electronic and Numeric Integrator And Computer), is brought
on-line. With it's activation, the foundations laid by the
ABC are built upon, and the Digital World is created. From the
ABC and ENIAC, various multiple Digital World realities grow,
which run parallel with the various multiple Earth realities in
existence in the multiverse.
This information comes from the "Brave Tamer" video
game. While continuity glitches between the games and anime mean
that the video games may or may not be in continuity, this game
explains that all the myriad "alternate universes" in
Digimon - "Digimon Adventure," "Digimon
Tamers," "Digimon Frontier," etc - all grew out of
ENIAC and the ABC. Hence, the ENIAC explanation can be taken as
canon for this and all timelines.
1948
The US and the UK make a secret agreement
to start intercepting communications. They start work on a SIGNIT
("Signal Intelligence") project.
1970
The very first computer virus, "the
worm," infects the US Department of Defence computers. It is
destroyed by the "reaper," a program designed and
created to delete excess data and thereby control the growth of
artificial intelligence in the world's computer network.
The 1980's
The US Department of Defense's SIGINT
system, known as "Project Echelon," is brought on-line,
with many communications bases in countries all over the world.
It's theorised that the reaper program is the basis for this
project's security system.
In various countries, attempts to create artificial intelligence
using computers are underway. The
result is the DigiGnomes, who grow from the matter of the digital
network world created by the ABC and ENIAC.
1984
Doctor Rob "Dolphin" McCoy begins to experiment in
creating artificial life, using drawings made by his son as a
basis for the creatures he designs. He is soon joined in his work
by students Janyu "Tao" Wong, Goro "Shibumi"
Mizuno, Rai "Curly" Aishuwarya, "Babel," and
"Daisy" (real names not known). Together, they form the
group called the Monster Makers - and create the beings known as
Digimon.
1986
Funding is withdrawn from the Monster Makers' project, and it is
abandoned. However, the data of their digital creations
accidentally finds it way into the digital plain created by the
ABC and ENIAC, and there, they begin to live and grow. The
"Digital World" is still small, but it grows with them.
1990 and onward
The Digital World exists as only one plain, where the early
Digimon and Digi-Gnomes live - but they soon come under attack
from the reaper program, which deletes many of them. However, it
absorbs the data of those that it deletes, and then enters a
period of inactivity, hiding itself away as it is mutated by the
absorbed data, attaining sentience and increasing in power.
Meanwile, in the real world, Shibumi continues to work on the
Digimon project on his own, determined to create a self-dependant
life form that can exist and evolve on it's own. He writes an
algorithm that will allow Digimon to evolve beyond their original
specifications. With the reaper gone, Digimon are free to grow
and evolve again.
1993
After a car accident, Shibumi goes into a form of coma, which
allows him to project his mind into the DigiWorld. At some
undetermined point following this, Shibumi sends a Mokumon to
Earth, where it assumes his form and carries out activities.
By 1994
Four Digimon have evolved to the highest stage, and become the
ruling "gods" of the Digital World - Azulongmon,
Zhuqiaomon, Ebonwumon and Baihumon, the four Digimon Sovereign.
The Digital World continues to grow, expanding into six plains,
populated with small "miniverses" - localised areas
specially created to suit the Digimon who inhabit them. Digimon
continue to Digivolve, thanks to the power of the Shining
Digivolution, a power source kept on the highest plain of the
DigiWorld, where the Sovereign dwell, which provides the energy
that enables Digimon to Digivolve.
1997
Digimon becomes a children's virtual pet-type computer game, and
immediately becomes a big hit.
E-mail interception by the US government
begins. It is believed that a Project Echelon intercepticon
facility is set up in Japan.
1998
Digimon becomes a card game.
1999
Public concern surrounding the invasions of
privacy perpetrated by Project Echelon is raised. The US
government denies it's existence.
Use of the network increases widely, and the Digital World
grows as the amount of transmitted information increases.
The first season of the "Digimon"
animated series is broadcast.
2000
The second season of the
"Digimon" animated series is broadcast.
2000-01
The Japanese government sets up it's own SIGINT organisation to
monitor the network, known as "Hypnos." The
organisation soon becomes aware of the Digimon, existing
autonomously within the network, and labels them "Wild
Ones," setting up protocols to deal with them as they
"Bio-Emerge" into the real world.
200X minus one
(note that "200X" is the officially-given
year for the events of the "Digimon Tamers" series,
meaning a year somewhere between 2001 and 2009)
Ryo defeats Rika in the Digimon card tournament of that
year. Subsequently, he goes to the Digital World, and stays
there, as his partner Digimon, Cyberdramon, is too feral to take
into the real world safely.
200X
The DigiGnomes use Shibumi's evolution algorithm to link humans
with Digimon. The algorithm manifests itself in the real world
via the cards of the Digimon game, making a "blue
card."
While playing the newest Digimon video game, Henry Wong, son of
Janyu Wong, comes to the conclusion that Digimon exist in a
reality outside the game. As he expresses remorse for hurting the
Digimon within the game, a blue card appears, transforming his
card reader into a D-Power Digivice, and bringing his partner,
Terriermon, to the real world.
Due to Ryo's absence, Rika claims victory at the year's Digimon
card tournament. Following this, Rika is confronted by a
multitude of Digimon, who all want her to tame them. She only
wants one, strong Digimon, however, and a blue card appears
before her, transforming her card reader into a D-Power Digivice,
which brings her partner, Renamon, to the real world.
Meanwhile, in the Digital World, the reaper program is
re-awakened by the sheer amount of excess data created by
Digivolving. The four Sovereigns are forced to take action as the
"D-Reaper" sets about deleting the Digital World,
carrying out it's original programming with extreme prejudice -
and the power to back it up. Azulongmon has the DigiGnomes
transform the Shining Digivolution, to hide it, and do away with
Digivolution all together, as it was what caused the problem in
the first place. The Shining Digivolution becomes Calumon, who is
then expelled from the Digital World, and winds up on Earth.
Following this, the events of the Digimon Tamers series
occur.
This information is mainly adapted from the "Chronicle" on Chiaki Konaka's website, translated (poorly, I might add!) by Altavista/Babelfish - although since this timeline's construction, the Chronicle has become available in English on his site (although it's still a bit... wordy). I've also inserted information that was given in the show. Also, some information was derived from various sites I found while searching for more information on SIGINT and Project Echelon.