The Valve Combined Universe Theory

WARNING: Do not attempt to read if you are not a fan of Valve Software. Doing so may cause brain damage. For those of you who are Valve fans, read this while temporarily suspending your disbelief, and believing something like TF 2 and it's artstyle can fit into Half-Life. Let's just say what we see in TF 2 is from the Demoman's POV: He's drunk.

In 1850; RED and BLU each own a massive amount of property in the world, and fight eachother constantly for land [2fort for it's waterways, Goldrush for the massive supply of, well, gold, etc.] RED hires 9 mercenaries to take over BLU's land, but Elizabeth tips off BLU to this, and they do the same. The first teams did not have respawn, and therefore, didn't last very long. Later, RED created a cloning system, creating self-aware clones. VOX, RED's supercomputer, realizes it would be shut down if the war were won, and delivers the schematics to BLU with the promise he would be backed up on BLU's database. The Administrator succeeds VOX, manipulating RED and BLU to hire a new team of mercenaries. RED eventually wins, and VOX gets pissed and launches a nuke, destroying 2fort and killing the Administrator and the Mercenaries. BLU's Spy manages to work out the relationship between the two companies and VOX, escaping before it can launch the nuke. The Spy, enraged that he looked like a fool, vows to take revenge on VOX. The Government notices the explosion and look into the two companies. They manage to secure RED and BLU holdings, but all attempts to secure Mann Co holdings are stopped by a muscular, Australian man. They later take inspiration from the two companies, assembling a team of "Counter-Terrorists" made up of the highest ranking SWAT team members to take down a league of terrorists overseas. What's left of RED and BLU go underground and change their names to Black Mesa and Aperture Science. Mesa uses their remaining money to establish a large scientific facility with VOX in charge, Aperture sets up a research facility with a backup of VOX being kept there.

With the Engineer dead, no one is around to maintain Redmond and Blutarch's life-extending machines, therefore, they died. They were succeeded by Wallace Breen and Cave Johnson, respectively. Johnson's first initiative as president of Aperture is to upgrade technologies created by the Engineer to a more efficient form, combining the Dispensers and mini-Sentries and experimenting with the Teleporters. In addition, he begins increasing the company's revenue stream by selling shower curtains and starting "Project: Richochet"; a reality show where contestants are immersed into a virtual world and attempt to knock eachother into the abyss with disc launchers for cash. Richochet is a huge success, and Aperture deactivates VOX, attempting to upgrade it to VOX 2.0, tentatively titled "G La DOS". Mesa begins working at a faster pace. Eventually, word gets to Black Mesa that Aperture is upgrading the Engineer's teleporters, Mesa begins developing a large scale version intended for transporting to Xen, a planet recently noticed by NASA. The Spy manages to infiltrate a NASA building and steal a large sampling of Xen Crystals and anonymously leaves them at Mesa's front door, knowing what they'll be used for. Spy makes his way to Engineer's former workshop to steal the blueprints for Blutarch's life-extending machine and the Australium needed to build it. At this time, Aperture Science completes the Handheld Portal Device and sets a tenative time for G La DOS's activation: The annual Bring-Your-Daughter-To-Work-Day celebration. Coincidentally, the day Black Mesa is scheduled to begin their experiments on the Xen Crystals.

On Bring-Your-Daughter-To-Work-Day, two catastrophic events happen at the same time. G La DOS is activated, and becomes a little nutty thanks to all the unfammiliar code within her [or his] files. She locks down the facility and slays all but Douglas Rattman, who happened to be investigating the chambers at the time; and Chell Johnson, planning to torture Chell. However, the testing data implemented by Johnson overrides VOX's, and she instead places Chell in a pod, cloning her as she did with the mercenaries. Rattmann eventually dies. In New Mexico, the Black Mesa Incident takes place, destroying the VOX being held at their base. Gordon Freeman attempts to stop an invasion, as the Spy watches. Impressed by his versatility, the Spy essentially kidnaps Gordon and drags him to a rebuilt life-extender and hooks him in.

The Government begins hunting down survivors of the Black Mesa incident, including Breen, Gordon, the Spy, etc. Sightings of Xen Lifeforms and Headcrabs become more and more frequent by the day, mostly covered up by the Government. Things got worse from here, however, as a complex life-form simply known as "the Infection" appear from the Portal Storms and combines itself with various rabies strands. Aperture is suffering massive profit losses and Mesa is nearly dead, it's every man for themselves from here on. The Army attempts to save people and kill all Carriers, a group of veterans [Day of Defeat] set up Milhaven, a central base for the Army. During this time, Portal takes place. They hoped scientists could create vaccines, but they were not able to. The Jockey is not really a Special Infected in the normal case, rather, what happens when a 'Headcrab Zombie' is infected, merging the the Headcrab DNA with the host's. Zoey, Francis and Louis manage to escape to a place known as "The Lost Coast", a former fishing town and vacation resort located in the Florida Keys. Rochelle, Coach, Nick and Ellis are essentially lined up against the wall and shot.

Afterwards, analysis of Portal Storms led to a stable enough cure for the Infection being found. It was dispersed around the world by jets, killing most of the human race with no way to change them back. Only very little Xen lifeforms remained, taking on traits the Infected used [Barnacles took on the Smoker's traits, etc.] Humans bunched together into small cities, being closely monitored by the government to prevent them from dying or causing chaos. Smartest Minds and Political Figures were protected in an old Black Mesa facility, where they discovered the TF 2 blueprints, which were ruined. Only clunky teleporters that took up a room could be built at this point.

Then, the 7-Hour War took place. The Spy sets Gordon free, the events of Portal 2 take place hundreds of years later, nothing notable from this point on.

TLDR; The Announcer from Team Fortress Classic AKA the Black Mesa Announcement AKA G La DOS and the Spy ruined everything.

WWII is rather.... insignificant?