Kenyan officials are worried about Somalia and the spread of Islamic radicalism.
Dusk falls over the thatched roofs and white walls of Lamu's ancient stone town. The lilting muezzin calls float out over the calm waters of the channel as the crooked alleys fill with Muslim faithful hurrying to evening prayers.
The idyllic scene has put the archipelago of Lamu on the itineraries of both well-heeled tourists and adventurous backpackers. X-Files star Gillian Anderson was married here last year, and Sex and the City's Kim Cattrall is a regular, drawn like many by the islands' relaxed "no news, no shoes" reputation.
It is a strange place to find the US military on active duty. The soldiers on patrol here are part of the 1,500-strong Horn of Africa Combined Joint Task Force stationed in a former French Foreign Legion base in the tiny Red Sea state of Djibouti.
The primary reason for the military manpower and its associated hardware lies just up the coast: the lawless, failed state of Somalia, long seen as a hotbed of Islamic fundamentalism.
"We find the terrorist networks here using the fact that there is a lot of ungoverned space in the Horn of Africa," said Maj Gen Samuel Helland, a former commander of the Djibouti task force.
"It's very easy for a terrorist organization to establish a presence. It's very easy for them to train, equip, organize, and use the facilities that are present to gain a foothold."
The border with Somalia is barely 70 miles north of Lamu, explaining why US officials were keen to accept Kenya's invitation to bolster their sea and land defenses against any southern spread of radical Islam.
American troops regularly support the Kenyan Navy as they search boats for drugs, equipment, or illegal agents trying to slip into Kenya.
They have on occasion marched in full combat gear through Lamu's narrow lanes in a show of force, especially during a tense period when investigators linked the area to a series of terror attacks in Kenya and Tanzania.
Fazul Abdullah Mohammed, the chief suspect in the 2002 suicide bombing of an Israeli-owned hotel in Mombasa and in a failed missile attack on an Israeli holiday jet, lived on one of the remote islands of the archipelago.