A month into the job in the governing British Conservative-led coalition government, Fox is in Washington for talks with U.S. Defense Secretary Robert Gates, other senior administration officials and members of Congress.
One of the toughest tasks confronting the U.S.-1ed military alliance is keeping it together in fighting the nine-year-old war. Britain has about 10,000 troops in Afghanistan, the largest international force after the United States, and has suffered 309 deaths.
More than 40 nations have troops on the ground.
"We must hold our nerve, maintain our resolve and have the resilience to see the job through," Fox said in the speech at the Heritage Foundation, a conservative policy think tank.
As the war winds on, the alliance faces the problem of keeping its members from dropping out. Australia and Canada are among those whose future role is uncertain.
"Our purpose is to degrade and manage the terrorist threat emanating from the region to ensure al-Qaida cannot once again have sanctuary in Afghanistan," Fox said.
Success, he said, would mean continuing to reverse the momentum of the Taliban-led insurgency and then reduce its threat to a level that the Afghan government can handle itself.
It is a gradual process, Fox said, adding: "There will be no decisive Napoleonic battle."
Significantly, perhaps, the British defense secretary did not speak in terms of a crushing all-out victory over Taliban and al-Qaida.
Nor did he minimize the stakes, one of which he said was protecting Pakistan, a nuclear-armed nation with a population of 168 million.
"Were we to leave prematurely," Fox said, without degrading the insurgency and increasing the capability of the Afghan National Security Forces, we would probably see the return of the destructive forces of transnational terror."
It also could risk the return of civil war and "would be a shot in the arm to jihadists everywhere," he said.
In response to a reporter's question, he said of the wavering nations, "it is essential to get our allies to understand our strength lies in common resolve."
And, he said, the coalition must keep up political pressure on the Afghan government to tackle corruption and improve its efficiency.