Warning: Spoilers. This article is nothing but theories and speculation. Do not use this as a reference for understanding the canon storyline.
The HL4D Problem
Half-Life and Left 4 Dead are probably the two most difficult stories to reconcile. The events of Half-Life take place in 200X, while the events of Left 4 Dead occur in 2009. The Half-Life series makes no reference to the occurrence of a zombie apocalypse, and the Left 4 Dead series makes no reference to the Black Mesa Incident.
We are given some breathing room due to physical distances. Left 4 Dead takes place in Pennsylvania, while Left 4 Dead 2 involves a journey through the southeast United States. The Black Mesa Research Facility is located in New Mexico, in the southwest United States, and, due to its isolated nature, would likely avoid most of the effects of the infection. We are given more breathing room in that we have no definite time frame on how quickly it took the effects of either incident to propagate. It is most likely that the Portal Storms began just as the infection began to wind down.
There is even more breathing room when considering the impacts of the spread of the infection and of the Portal Storms following the Black Mesa Incident. The Portal Storms preceding the Seven Hour War were notable in their devastation of rural areas and encouraging humanity to relocate to more urban areas, as cities were more well equipped to handle the infestation of alien lifeforms. In direct contrast to that, the infection was particularly devastating to urban areas due to its contagious nature, encouraging people to spread out into areas of lower population density.
Posters and maps on saferoom walls in Left 4 Dead imply that much of the United States is overrun by the infection. It provides no information on whether the infection was an epidemic restricted to North America or a global pandemic. The most reasonable assumption would be that the epidemic was indeed restricted to North America and consequently overshadowed by the much larger and more devastating Seven Hour War.
From this, we begin to see a possible picture of the situation. City 17, the seat of Wallace Breen's world administration, is located in Eastern Europe. Why should the global capital have been set in that region? Perhaps the long-term devastation of urban areas in North America by the epidemic had led to this arrangement. Following the devastation of the urban areas of the United States, people fled the country or sought refuge in the rural areas. Then, the Portal Storms swept over the world, wiping out what remained of humanity still hiding in rural North America. As a result, the infection was essentially gone by the time the Combine arrived.
Ellen McLain
The TF Announcer (also known as The Administrator) of Team Fortress 2, GLaDOS of the Portal series, and the Overwatch Dispatcher of the Half-Life 2 series are all voiced by Ellen McLain.
Theory: As the war between RED and BLU began to wind down in the 1970s and 1980s, either the Announcer or some relative eventually entered the employ of Aperture Science and became the base on which the "Genetic Lifeform" part of the "Genetic Lifeform and Disk Operating System (GLaDOS)" was built. Being an artificial intelligence program, GLaDOS could be copied and replicated, and a copy of GLaDOS entered Combine possession after the Seven Hour War. The Combine tamed and modified the AI program and incorporated it into their system, and what remained of the GLaDOS program became part of the Overwatch Dispatcher.
Support: The Combine are known for incorporating the features and technology of conquered species into their own while assimilating them, as this allows them to better adapt their control on a planet-by-planet and universe-by-universe basis. On Earth, helicopters and trains modified by Combine technology are still widely used. In the military wing, the Combine submachine gun appears to be based off the MP7 and the shotgun appears to be based off of the SPAS-12. Even the AR2 pulse rifle incorporates human gun technology in order to allow the transhuman Combine soldiers to continue to use them. It therefore seems plausible that the Combine might incorporate a successful human-made AI program into their own AI system.
Appeasement
Neville Chamberlain and several other European leaders agreed to allow Germany to annex the Sudetenland, believing this would ensure peace after being assured by the German leadership that their expansion would end there. When Germany later invaded Poland in 1939, war broke out. The Soldier would later buy his own plane tickets to Europe to join the fight after being rejected from all branches of the United States military, starting a rampage in Poland that would not end until 1949.
Theory: The Gman or the faction he works with convinced the British government that German expansion would indeed end at Czechoslovakia. In doing so, He or They ensured that the war in Europe would break out not over Czechoslovakia but over Poland. This in turn ensured that the Soldier would spend most of his personal campaign in Poland, where he would learn the fighting skills necessary to be successful in his future employment in 1968.
Support: The Gman and his supposed faction are known manipulators. At the same time, it is interesting that the Soldier would reportedly be interested in fighting in Poland when the United States military was focused on France. The Poland was largely left to the Soviet Union, while much of the British focus was on Italy and the Balkans. It is plausible that Poland's status as being the start of the war in Europe attracted the Soldier there, and his presence in Poland would ensure his separation from American and British operations.
