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Since the story is still in the process of being written, there is always a chance that facts will be altered at a later date. Valve's official stance is that they do not intend on providing a definitive Half-Life / Portal series timeline as they fear the possibility of contradicting themselves would limit their creativity in future games. That timespan saw the rise and fall of rival scientific research corporations Black Mesa and Aperture Science, the invasion of Earth by the Combine, an alien intergalactic organization, in the aftermath of the Black Mesa Incident, and the uprising of humanity against them.
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The timeline of the Half-Life and Portal universe spans from the 1940s to the first decades of the 21st century. Freeman? Is it really that time again? It seems as if you only just arrived." ― The G-Man
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