The events of Annihilation saw the universe consumed by war and many of Marvel's cosmic heroes brought to their knees by Annihilus and his Annihilation Wave. Though heroes such as the Guardians of the Galaxy, Silver Surfer and Nova didn't know it yet, things were about to get much worse. Capitalizing on the weakened state of the cosmos, the villainous automaton Ultron decided to try and conquer the vulnerable galaxy. Powered up immensely by a technocratic alien race known as the Phalanx, he set about consuming the universe, much like Annihilus before him. First encountered the X-Men, the Phalanx almost assimilated Earth before they were ultimately betrayed by one of their human allies, Steven Lang.
Annihilus' plan for the universe was simple: he wished to consume all life in the universe out of fear. Terrified of his own end, he sought to remove all other life to eliminate any chance that anyone could kill him. While this plan seemed a little dramatic -- considering Annihilus can be reborn from a larval state in the event he is killed -- the very thought of perishing shook him to his core. So, he summoned an impossibly large fleet of ravenous space bugs known as the Annihilation Wave and even took Galactus prisoner (with the help of Thanos) to power his doomsday machines. Thankfully for the universe, he was defeated when Nova pulled his insides out.
Following the initial Annihilation war, the Kree Empire was in need of rebuilding. Many heroes helped, including the Space Knights, the unknowing carriers of the universe's next blight. The Phalanx, able to assimilate organic life into their grand consciousness, piggybacked on the Space Knights and infected the Kree homeworld of Hala. Using Hala as the base for a new conquest, the Phalanx erected a forcefield around all of Kree space and infected it entirely within weeks. Star-Lord formed a resistance on Hala which included Groot, Bug, Mantis, Rocket, Captain Universe and a few remaining uninfected Kree. Elsewhere, Quasar, Moondragon and Adam Warlock appealed for help from the High Evolutionary, only to be ambushed by Korath the Pursuer.
Korath failed to obtain Adam Warlock for his master, who was revealed to be the Avengers villain Ultron, and was executed. Although, Korath wouldn't be the only cosmic hero to be assimilated by Ultron, as over the course of his crusade the villain would bring some real heavy hitters to his side using the Phalanx. He infected Nova, Gamora, Blastaar and Drax in order to help him crush Star-Lord's resistance. One of his aims was to assimilate and duplicate Adam Warlock, to eradicate Earth, showing he still harbored a desire to defeat the Avengers. Luckily for the allied forces, the mutant Warlock (of the technocracy also) was able to remove Phalanx infection from several heroes.
In retrospect, the plans of Annihilus seem to be more clearly motivated by fear and less calculated than Ultron's. Where Annihilus just wanted to be rid of all other life through fear of his own mortality, Ultron's machinations with the Phalanx showed a more calculated and sadistic endgame. The Phalanx's use in his schemes gives us a chilling look into what might have been for the galaxy -- given what we know now about their relationship to humanity and the X-Men -- as seen in House of X/Powers of X. In the far future, the Phalanx are a huge machine consciousness of knowledge made up of several civilizations. These civilizations have ascended to a point where organic needs and functions are no longer necessary and humanity is set to eventually join them, by their own request.
For Ultron, however, there is no request from his victims. He took the phalanx and their ability to welcome other beings into their network and used it to enact his sinister designs. It allowed him to add beings to the Phalanx that might not have been destined to join them for eons yet. This mashing together of flesh and machine was far more insidious than Annihilus' Annihilation Wave, as it not only brought strife to the universe in the present but was a direct perversion of the evolutionary progress of the galaxy. Though he would be stopped by Quasar, Ultron yet lives and is coupled together with the organic image of his creator Hank Pym in a horrifying amalgamation.