Ba'al, having used the Solar observation outpost on the planet Praxyon to travel back in time to 1939 and prevent the Achilles from reaching the United States of America with the Stargate, was able to change the course of history and prevent the Stargate Program from ever existing.
After using his knowledge of the future to take control of the Goa'uld Empire by offering the Jaffa freedom in exchange for helping him defeat his enemies. After killing Apophis, the last System Lord to stand against him, he set his sights on Earth. (SG1: "Continuum")
Ba'al later sent a massive fleet to Earth in an attempt to subdue them. His plan was to use a soft touch and persuade them to join forces and serve the Goa'uld, but the rest of the Goa'uld protested, and Ba'al was killed by Qetesh, who took his place. She ordered Cronus to fire down on the planet, stating that more of the planet Cronus had destroyed, then the more Cronus would be richly awarded for his efforts. The attack began and several major cities, including Washington D.C., and largely populated areas around the world were fired upon. Death Gliders were sent down to the planet to destroy Earth's aircraft. Major battles were fought all over the world, presumably resulting in the Earth being destroyed or conquered with numerous fatalities as well but in the end, time was reset by Colonel Cameron Mitchell and the battle never took place. (SG1: "Continuum")