I don't own SG-1 or Atlantis, which is probably a good thing. No copyright infringement intended.
This is an SG-1/SGA AU starting with the events of the series 5 episode 'Revelations'.
Adara II, Milky Way Galaxy
Three bursts of Cherenkov radiation and pseudomotion signalled the arrival of three large, tetrahedral vessels into the system. On the bridge of the lead vessel, a deep, flanged voice gave orders, and the vessels moved to establish themselves in orbit over the world beneath. More orders, and three powerful sensor arrays began probing the atmosphere and surface of the planet.
What the arrays did not discover, however, was the cloaked satellite in high orbit, which sent a preprogrammed, heavily encrypted subspace signal to it's extragalactic owners when it detected the three vessels.
The planet was somewhat of a mystery to the masters of these vessels, as it was uninhabitable to them, their human hosts and their diminutive but very powerful enemies, yet it had been included in the Protected Planets Treaty. The System Lords had suspected for some time that the world held special significance in the eyes of their enemies, but had never dared to provoke their wrath by investigating.
These were Goa'uld ha'tak in the service of Anubis, one of the most ancient of the Goa'uld, and now one of the most powerful, thanks to Anubis' ascension. Osiris had been dispatched to investigate the planet, and if any enemies were found, they were to be destroyed, to make an example of those who would defy their true gods. Such was the way of the Goa'uld, a way that had served them well for thousands of years, and naturally the Goa'uld thought it would serve them for thousands more. Osiris was settled in his throne, idly watching the progress of the scans, passing time by tormenting his host with images of the inevitable enslavement and destruction of her world. His attention was brought swiftly back to the present, as the Jaffa operating the sensors addressed her.
"My Lord, our sensors detect a vessel emerging from hyperspace," he reported, with the due deference to his gods clear in his voice. "It's mass is consistent with that of an Asgard vessel."
"Very well. It would seem that we are unwelcome here. Bring the other ha'tak into formation, raise the shields and arm all weapons. We shall confront this intruder into our Lord Anubis' domain," Osiris watched as the Jaffa complied smartly with his orders, and within a few moments the Asgard vessel was within visual range. Osiris could see that it was a Beliskner-class vessel. Easy prey for his ships, he thought.
The Jaffa at the helm reported that the Asgard was attempting communication. "Put him on screen," Osiris ordered, rising imperiously to his feet. An Asgard face duly appeared on the screen.
"I am Thor, Supreme Commander of the Asgard fleet. Your presence here is a violation of the protected planets treaty , and you must withdraw immediately," stated the grey alien.
"I don't think so," Osiris retorted smugly.
"If you do not comply, I will be forced to open fire," Thor warned. Osiris' smirk grew wider.
"Yes, I suppose you would!" he sneered.
Thor's reply was almost disbelieving. "You will be destroyed," he said.
Osiris openly laughed at the alien. "I am the great god Osiris, and gods cannot be destroyed!" Osiris' tone became softer, more deadly. "Surely you should know that!"
Thor's reply was resigned, sad almost. "Very well," he said softly. Immediately he fired upon Osiris' hat'ak with weapons that should've destroyed the Goa'uld ship easily. Instead, he was stunned to see the enemy's shields hold fast under the barrage of energy bolts.
Just before he returned fire, Osiris' reply was chilling in it's finality. "Our days of bowing to the Asgard are finally over," he said imperiously.
Too late, Thor realised his mistake, and attempted to flee into hyperspace, only to be struck by a barrage of plasma fire which ripped the shield generators from their mountings, smashed his hyperdrive systems and caused the reactors to shut down. Thor barely had time to send a distress call back to the High Council before his ship was completely disabled. Thor sighed. "That could have gone better," he lamented, in a manner not dissimilar to that of his Tau'ri friends.
As Goa'uld transport vessels approached his stricken ship he wondered how he was going to get out of this particular situation alive, for the Asgard had no other ships in this galaxy to spare for his aid...
Well, there we are, first chapter. What do you think?
This was just the prologue, the next chapter is more AU than this one.