The Skirmish in Germany was an attempt made by the Loki, with Hawkeye under his thrall, to steal Iridium in Germany in order to stabilise the Tesseract. His plan quickly developed into a brief battle after the Avengers intervened.
When Loki, after being exiled from the Nine Realms, encountered The Other, he was assigned to retrieve the Tesseract for a cybernetic race known as the Chitauri. At the Joint Dark Energy Mission Facility on Earth, the Tesseract acted up, forcing the facility to be evacuated.
Loki appeared on Earth due to the acts of the Tesseract and possessed the minds of scientist Erik Selvig and S.H.I.E.L.D. agent Clint Barton (Hawkeye). Loki and his now-possessed agents escaped the facility.
Nick Fury and Phil Coulson, who were present at the evacuation, recruited Tony Stark, Bruce Banner, Steve Rogers and Natasha Romanoff for the Avengers Initiative. Fury considered Thor Odinson, but found out that he was far from Earth.
Loki and Barton came to a point where they needed Iridium to stabilise the Tesseract. They created a plan in Loki's Hideout.