2009 Sontaran invasion of Earth

The Sontaran invasion of Earth was the attempted takeover of Earth by the Sontarans for the purposes of breeding more clone warriors.

Background
During the early 21st century, the Sontaran Empire was losing its war with the Rutan Host. They were running out of soldiers and needed to produce more. They decided that a large planet such as Earth would be a good target and set about their plans to eradicate humanity and convert Earth into a cloning factory. (TV: The Poison Sky)

Beginnings
Part of their plan called for Luke Rattigan to market ATMOS devices across Earth. These would be activated at the appropriate time. They did so shortly after the Unified Intelligence Taskforce's storming of the ATMOS factory. After UNIT had taken over the factory, Martha Jones was captured and cloned to become an agent for the sontarans. When the ATMOS devices were activated, they pumped a thick gas into the atmosphere which had two purposes — to poison humans and to aid the growth of Sontaran clones. (TV: The Sontaran Stratagem)

At this point, many humans died — the first deaths happened in Tokyo, Japan. As other urban areas filled with smog, refugees escaped to more rural areas or even into the oceans. (TV: The Poison Sky)

Battle
UNIT prepared many plans for the Sontarans, including firing nuclear weapons at the Sontaran flagship. This attack was thwarted by a clone of Martha Jones, who was given a device to wipe the nuclear launch codes and the order to do so repeatedly. This was not because it would damage the Sontaran ships — it wouldn't — but because it would ignite the clonefeed in the atmosphere and end the invasion.

Around this time, a Sontaran attack squad retook the ATMOS factory from UNIT and killed all the humans in the area. Shortly after, UNIT prepared its soldiers from one of its nearby bases and used the Valiant to clear the smog from the area and shoot the factory, killing several Sontarans. Entering the factory once again, the human soldiers swiftly decimated the Sontaran attack squad. (TV: The Poison Sky)

Sontaran defeat
The Tenth Doctor ignited the smog with Luke Rattigan's atmospheric converter and flames spread across the Earth. When they died down, the planet's atmosphere was restored to its previous state. The Sontarans prepared to attack the Earth with its fleet. However, Rattigan sacrificed himself by using the atmospheric converter on the flagship. He exploded the flagship; its fragments destroyed the rest of the fleet. (TV: The Poison Sky)

One Sontaran pod, piloted by Commander Kaagh, survived the fleet's destruction. When the flagship was destroyed, his pod was knocked into space by the explosion. It crash-landed on Earth, where Kaagh staggered out with a scarring gash across the right side of his face and part of his right ear mangled. (TV: The Last Sontaran)

Aftermath
Many humans were afraid to travel by car (as it had been cars that the ATMOS devices were connected to). Other than that, life on Earth apparently returned to normal. (TV: The Poison Sky) UNIT took advantage of the teleport pods left behind in the ATMOS factory and the Rattigan Academy. This led to Project Indigo, which would prove useful during the War in the Medusa Cascade. (TV: The Stolen Earth)

Commander Kaagh, the lone survivor of the Tenth Sontaran Battle Fleet, crash-landed near Goblin's Copse. Though severely injured, he repaired his ship in secret in the woods near the Tycho Project radio telescope. His drones looked like lights in the sky, attracting human attention. Kaagh used the drones to lure Professor Nicholas Skinner and his daughter out of the radio telescope. He seized them and fitted them with neural implants to control their actions. He put Professor Skinner to work to make all of Earth's satellites crash onto nuclear stations. This would cause a chain reaction and destroy the Earth, avenging the loss of his comrades-in-arms. His plans were foiled by Sarah Jane Smith, her son and his friends. Kaagh was sent home to Sontar in shame. (TV: The Last Sontaran)

Behind the scenes

 * The invasion of modern London by General Staal is mentioned by Commander Strax in the fourth-wall-breaking cinema intro for The Day of the Doctor. He is disappointed to learn that the anniversary feature is about the Doctor as opposed to a Sontaran historical drama featuring heroic accounts of their various military campaigns which he was anticipating with his clone batch.