Protected Planets Treaty
An agreement established by the Asgard High Council and the Goa'uld System Lords. The Goa'uld agree to leave alone all planets included in the treaty. The Asgard protect the planets from direct Goa'uld attack, but agree to otherwise not intervene in any other threats to the worlds. In addition, planets included may not be allowed to advance technologically to a point where they may be a threat to the Goa'uld.
The planets protected by the treaty include Earth, Cimmeria, K'Tau, and many others -- 27 in total (as of "New Order, Part 1"). In exchange for Earth's inclusion, the Asgard granted Goa'uld ships access to the Passage of Nylor. The Goa'uld also demanded that Earth's development be limited, and attempted (unsuccessfully) to seize both of the planet's Stargates.
Thor's Chariot - SG-1 learns that the planet Cimmeria is under Asgard protection -- thus the hammer device that the team destroyed a year earlier.
Fair Game - The Asgard and the Goa'uld System Lords arrive on Earth to negotiate its inclusion into the Protected Planets Treaty.
Red Sky - SG-1 finds the Asgard-protected planet K'Tau, but bring doom to its people by damaging their sun. The Asgard refuse to invene, in keeping with the treaty, which prevents them from saving a world from a non-Goa'uld threat.
Between Two Fires - The Goa'uld attempt to destroy Earth and avoid violating the treaty by forcing the Tollan to send a bomb through the gate to Earth -- one capable of penetrating the iris.
Last Stand - The System Lords agree to circumvent the Protected Planets Treaty by allowing Anubis to destroy Earth before he is admitted into their ranks.
Fail Safe - The Protected Planets Treaty essentially nullified, the Goa'uld (presumably Anubis) hurtle a naquadah-enriched asteroid toward Earth.
Disclosure - As a thanks for helping in the struggle against the Replicators, the Asgard equip Earth's first interstellar vessel with Asgard weapons and shields -- apparently acknowledging that the treaty with the Goa'uld is null and void.
New Order, Part 1 - The System Lords tell Earth that Baal is threatening to take the planets in the treaty for himself, believing that the Asgard no longer wield power in our galaxy.