Mr. Ahmed, who was sent to New York on Saturday after being arrested in the West African country of Nigeria, is charged with providing and conspiring to provide material support to the militant Somali group, Al Shabab, which the State Department has listed as a terrorist group.
He's also charged with receiving and conspiring to receive military-type "jihad training" from Al Shabab, which has recently sworn allegiance to Al Qaeda.
Eritrea is the bitter enemy of Ethiopia, which occupied neighboring Somalia for more than a year after a US-backed Dec. 2007 invasion to wrest control of the country from militant Islamists.
In an enemy-of-my-enemy-is-my-friend calculation, Eritrea has for years supported Al Shabab, at least according to the US government and most independent Western analysts.
During her trip to Africa in August, Secretary of State Hillary Clinton warned that the US would "take action" against Eritrea if it does not stop supporting Al Shabab's efforts to overthrow the weak, Western-backed transitional government in Somalia.
"It is long past time for Eritrea to cease and desist its support of Al Shabab and to start being a productive rather than a destabilizing neighbor," she said. "We are making it very clear that their actions are unacceptable."
Mrs. Clinton also said then that if Al Shabab obtained a haven in Somalia "it would be a threat to the United States." The group controls wide swaths of the Somalia.
All of which could play a role in why Ahmed is being charged in New York.