The Protected Planets Treaty is an agreement between the Asgard High Council and the Goa'uld System Lords. The Goa'uld view technologically advanced planets as a threat, however, in fear of the Asgard they reluctantly agreed to include certain planets within the restrictions of the treaty. The treaty protects these planets from Goa'uld domination and attack by the System Lords, who must enforce the treaty upon all Goa'uld. In exchange, Subsection 42 states that the people on these planets cannot be artificially advanced through Asgard technological means. Thus, interference from the Asgard, even for natural disasters, would nullify the whole treaty and leave all protected planets vulnerable to attack.
Cimmeria and K'tau are protected planets, and Thor acted as a representative of the Asgard High Council to mediate negotiations with Cronus, Nirrti, and Yu for Earth's inclusion in the Protected Planets Treaty. The initial version of the treaty included Section 326 which recognized that humans exist for the purpose of serving the Goa'uld as hosts and slaves, and that no human planet should be allowed to advance technologically to a point where they may become a threat to the Goa'uld. It also stipulated that Earth forfeit both Tauri stargates. In exchange for exposing Nirrti's attempted murder of Cronus and her sabotage of the negotiations, however, the terms were altered to remove those demands, and the treaty was finalized. The treaty protected Earth from Goa'uld attack but did not protect the Tauri in encounters beyond Earth.
With the rise of Anubis, however, the treaty has been repeatedly violated. By attempting to use Tollan phase-shift technology to send a weapon of mass destruction through Earth's iris, by placing a naquadah asteroid on a collision course for Earth, and by developing a weapon that would destroy Earth by using a power overload within the stargate, Anubis repeatedly attempted to destroy Earth from outside the terms of the treaty. As their war with the Replicators became more critical, the Asgard were left with limited available resources to enforce the treaty, and as the Protected Planets Treaty became largely invalid, the 27 protected planets, including Earth, became vulnerable. Following the destruction of Anubis, Ba'al began consolidating his power with the intention of claiming the protected planets for himself. With the demise of the treaty, the Asgard were no longer bound by its noninterference clause, and they agreed to offer Earth their assistance and technology to help in the fight against the Goa'uld.
Cross Reference: Adara System, Anubis, Anubis's Ancient Weapon, Asgard, Asgard High Council, Asteroid, Camulus, Cimmeria, Cronus, Freyr, K'tau, Nirrti, Arthur Simms, System Lords, Thor, Yu
Episode Reference: Fair Game, Double Jeopardy, Red Sky, Between Two Fires, Last Stand, Fail Safe, Revelations, Redemption, Disclosure, New Order