In 1998, a team of explorers from Stargate Command--SG-1--traveled through the Stargate to the planet designated P3X-187 to explore a potential mining source of the rare element Naquadah. Shortly after arriving on the planet, they were bombarded with enemy fire and were forced to extract themselves--but not before hearing a planet-wide announcement, presumably being made from the air:
Attention, all planets of the Solar Federation. Attention, all planets of the Solar Federation. Attention, all planets of the Solar Federation. We have assumed control. We have assumed control. We have assumed control.
An invasion was underway, but the SG-1 was able to escape.
Now, almost one year later, Stargate Command recorded an Unscheduled Off-World Activation of the wormhole, and a radio message was relayed through it. A local named Jon relayed that the invasion had failed, but it had given his race hope. They've been kept as slaves for hundreds of years by a Goa'uld named Syrinx--and they want help from the SGC. In return, they will negotiate mining rights to the planet's naquadah ore.
It fits perfectly within two of the SGC's standing orders: to obtain alien technology to help protect Earth, and to help liberate the peoples of the galaxy from the Goa'uld threat. SG-1 is on a mission, but the newly minted SG-10 is awaiting their first assignment.
Sg-10, you have a go.