Obama notified Congress of his order Thursday.
: "I hereby determine that it is necessary to augment the active Armed Forces of the United States for the effective conduct of Operation United Assistance, which is providing support to civilian-led humanitarian assistance and consequence management support related to the Ebola virus disease outbreak in West Africa."
The Pentagon said it had no immediate plans to send reservists or National Guard troops to Africa, saying that the order simply allows the military to begin planning for those forces in its overall response.
It "doesn't mean that we are deploying these forces, but it gives us the option to do so if we need to," said Air Force Lt. Col. Thomas Crosson, a Pentagon spokesman.
The White House said it didn't know exactly how many reserve troops would eventually be required.
"The president has laid out very clearly what the mission is. The Department of Defense has told the president that it will require about 4,000 Department of Defense personnel to execute the mission the president has directed them to execute," White House Press Secretary Josh Earnest said earlier in the afternoon. "What I don't know is the composition of that force."
Obama canceled a second day of political events Thursday in order to deal with the Ebola crisis from the White House. He was meeting with top advisers on Ebola Thursday afternoon and was expected to make a brief statement afterward.
The White House also expanded on its justification for not instituting a travel ban on people from Ebola-ravaged countries Thursday as pressure from lawmakers mounted to take that action.
"It would provide a direct incentive for individuals seeking to travel to the United States to go underground and to seek to evade this screening and to not be candid about their travel history in order to enter the country," Earnest said.
That's a new argument for the White House, which has all along argued that free travel is necessary in order to get health care workers and equipment from the United States to the African countries of Sierra Leone, Guinea and Liberia.