See also, The Seven Hour War
The Seven Hour War obviously only lasted seven hours, so there is not much to it. Basically, portal storms were created after the Nihilanth was destroyed, which drew the Combine to Earth. In seven hours, the Combine had completely annihilated the entire military force of the Earth. Dr. Breen managed to save the Earth from destruction by negotiating its surrender. The Combine rewarded him with power, and Breen now controls the remainder of the human race. The Combine have managed to suppress the humans’ reproductive cycles for at least 20 years, which explains why there are no children in City 17.