Seven Hour War

The Seven Hour War was a brief but decisive confrontation between the Combine and the governments of the Earth, ultimately leading to a large human death toll and the Combine occupation of the entire planet.

Overview
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 cork-board featuring several annotated newspaper cutouts and images causes him to reminisce over the events during the war. In Episode Two, Eli states that if the super-portal matures "it'll be the Seven Hour War all over again... except this time we won't last seven minutes!"

After the war ended, the Resistance was formed to combat the new Combine leadership.

Battle
The Portal Storms raged across the Earth, spreading Xen life forms causing the people to move into cities for protection. These portal storms attracted an alien empire known as the Combine that defeated all of Earth's armed forces in only seven hours. It was after this that Dr. Wallace Breen negotiated a surrender on behalf of the United Nations, "saving" the survivors at the cost of their freedom. Breen was appointed ruler of Earth, but remains under the control of the Combine. The official surrender appears to have taken place in New York City, in the United Nations building.

At the beginning of Half-Life 2, the G-Man makes a brief reference to the war, telling Gordon Freeman that "all the effort in the world would have gone to waste." This is generally understood to be a direct reference to the war, the establishment of Dr. Breen's terror state, and the reasons for Freeman's stasis. Despite Freeman's triumph at Black Mesa, there was nothing that he or the entire human race could have done to save Earth from the superior forces of the Combine empire.

Half-Life 2 begins an undisclosed number of years after this 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 preventing any further reproduction for approximately 20 years until the events of Episode One, with the suppression field being disabled after the damage the Citadel had taken.