Kenya has in the past trained Somali troops for deployment near the border to create a buffer zone in an effort to keep al-Shabab militants away, but the presence of Kenyan soldiers inside Somalia has always been played down or even flatly denied.
The very public announcement of this incursion into Somalia by the Kenyan army and air force is a significant change of policy, which seems at least partly aimed at sending a message to the Kenyan people and the rest of the world that the issue of security is being taken seriously.
"Our territorial integrity is threatened with serious security threats of terrorism. We cannot allow this to happen at all," Kenya's Internal Security Minister George Saitoti told the media.
"It means we are now going to pursue the enemy, who are the al-Shabab, to wherever they will be, even in their country."
The operation - unprecedented for Kenya's largely inexperienced military - appears to have been triggered by the recent kidnappings of four foreign nationals and one Kenyan.
Two of them were taken on the north Kenyan coast and the others in the Dadaab refugee camp near the border.There is, however, no concrete proof that these kidnappings were carried out by the Islamist insurgent group.
In an area plagued by guns and Somali bandits, blaming al-Shabab could be jumping to a convenient conclusion.Some analysts think the military operation is not about rescuing the hostages but is actually about securing the border
Going after a group linked to al-Qaeda and which is on the US terror list is an easier sell to the public than chasing bandits inside Somalia.