Under Renovation

The Seven Hour War was a brief but decisive confrontation between the Combine and the governments of Earth, ultimately leading to a large human death toll and the Combine occupation of the entire planet. The war itself is never discussed directly in Half-Life 2, but it is mentioned briefly by Eli Vance in Black Mesa East. Looking at a small corkboard featuring several annotated newspaper cutouts and images, including a photo of a visibly damaged United Nations Secretariat Building in New York City, causes him to reminisce over the events during the war. In Episode Two, Eli later states that if the superportal matures, "it'll be the Seven Hour War all over again... except this time we won't last seven minutes!".

Very little is known about the war except that it began some time after the Portal Storms which raged across the Earth. These disturbances spread Xen life forms across the world, causing populations to move into cities for protection. Attracted to Earth by the dimensional rift caused by the Black Mesa Incident, the Combine, a vast alien empire, launched an invasive force that overpowered the planet, catching the human population by surprise and defeating all of its armed forces in only seven hours.[1] It was after this that Dr. Wallace Breen, former Black Mesa Research Facility administrator, negotiated a surrender, leading to a subservient and meager existence for Earth's remaining human population under the oppressor and "saving" the survivors at the cost of their freedom. Breen was appointed administrator of Earth but remained under the control of the Combine. Some time after the war, the Resistance was formed to combat the new Combine leadership.

Half-Life 2 begins an undisclosed number of years after the war & after the rule of the Combine has been firmly established. There is, however, some evidence from the Half-Life 2: Episode One official website indicating that Half-Life 2 takes place nearly two decades after the Black Mesa incident. Such a time frame is logically consistent with the aging of the series' characters between Half-Life and Half-Life 2. It also explains why there are no children in the game, the Combine having set up a reproductive suppression field a short time after the war to prevent any further reproduction for approximately twenty years until the events of Episode One, the suppression field being disabled after the damage the Citadel had sustained.

The exact manner in which the Combine forces invaded and conquered Earth during the war has not been revealed, but Half-Life 2: Raising the Bar describes a concept where Citadels were to be teleported directly to Earth. Once they had landed inside human cities, they began deploying Combine armies from within.

The Source[combineoverwiki.net]

- Ethan