This helpful BBC timeline reminds us that back in late December 2006, Ethiopian troops entered Somalia and remained until the beginning of 2009. The African Union has a small mission deployed to Mogadishu (AMISOM) and many governments, including the US, play an important part in backing the Transitional Federal Government (TFG) and the development of its security forces.
Heavily armed Ethiopian troops with several army trucks have reportedly crossed the border into central Somali regions of Hiran and Galgadud, residents and reports said.
Residents of Balanbale town in central Galgadud region said they have seen Ethiopian military forces backed by army vehicles in the outskirts of the town.
One resident said the troops have dug trenches in positions without prior notice of the elders.
Ethiopian troops have also crossed the border and reached Kalabeyr town in Hiran region, about 22km (14 miles) from the Somali-Ethiopian border, according to locals.
A resident told Garowe Online "a lot of troops arrived in the area on early Saturday and have started making military manoeuvring."
So, how long will the troops stay this time? How far into the country will they go and what impact will their presence have?