The Islamic Courts Union was an organization that came together as a result as a result of the chaos in Somalia in the 90s and early 2000s. As the name alludes to, they started out as religious courts, which banded together so they could stand up to warlords and enforce their rulings. Due to popular grassroots support they managed to wrest control of much of the country from the warlords, including the capital.
Unfortunately, this was not to the liking of Ethiopia, which worried that a strong Somalia would try to do Ogaden War 2: Islamist Boogaloo. So they invaded to remove the organization in favor of the official transitional government. They were supported by the US, since Ethiopia is a strategic partner and the ICU kind of looks like the Taliban if just skim the briefing (both were grass roots Islamist organizations that formed as a reaction to warlordism).
While the ICU was driven from Mogadishu and effectively shattered as an organization, the militant wing would reform as Al Shabaab, so it wasn’t exactly a win.
Taken from the Reddit thread: