A U.S. soldier watches a football match between British and German troops commemorating the Christmas Truce of 1914, at the ISAF Headquarters in Kabul December 24, 2014. British and German soldiers deployed in Afghanistan gathered in a dusty field on Wednesday to play a game of football in memory of the Christmas Day truce spontaneously called between their armies during the First World War in 1914. (Photo by Omar Sobhani/Reuters)

German and British troops participate in football match to commemorate the Christmas Truce of1914, at the ISAF Headquarters in Kabul December 24, 2014. (Photo by Omar Sobhani/Reuters)

British fans react during a football match between British and German troops to commemorate the Christmas Truce of 1914, at the ISAF Headquarters in Kabul December 24, 2014. (Photo by Omar Sobhani/Reuters)

German and British troops sing "Silent Night" before playing a football match to commemorate the Christmas Truce of 1914, at the ISAF Headquarters in Kabul December 24, 2014. (Photo by Omar Sobhani/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment embrace after attending a memorial for Specialist Wyatt Martin and Sergeant First Class Ramon Morris at Bagram Air Field in the Parwan province of Afghanistan December 23, 2014. Specialist Martin and Sergeant First Class Ramon were killed on December 12 by an improvised explosive device while on patrol near Bagram Air Field. (Photo by Lucas Jackson/Reuters)

German soldiers react as a helicopter takes off after a football match commemorating the Christmas Truce of 1914, at the ISAF Headquarters in Kabul December 24, 2014. (Photo by Omar Sobhani/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment prepare to fire mortars during an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

A U.S. soldier from the 3rd Cavalry Regiment waves the smoke from the opening of a mortar during an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment joke around with each other during an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

A U.S. soldier from the 3rd Cavalry Regiment carries a box of mortar rounds after an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

A U.S. soldier from the 3rd Cavalry Regiment returns to his quarters after an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment relax in their quarters after taking part in a mortar exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment relax in their quarters after taking part in a mortar exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment fire a 120mm mortar during an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment fire a 120mm mortar during an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment clean a mortar during an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment relax in their quarters after taking part in a mortar exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldier from the 3rd Cavalry Regiment take part in a Christmas Eve celebration with soldiers from the Polish army's 21st Mountain Brigade on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment pass out care packages and Christmas stockings to fellow soldiers and workers on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment take part in an artillery exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment prepare for an artillery exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment fire a 120mm mortar during an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment take part in an artillery exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment wrestle to pass time while waiting for an artillery exercise to begin on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldier from the 3rd Cavalry Regiment take part in a Christmas Eve celebration with soldiers from the Polish army's 21st Mountain Brigade on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment prepare for an artillery exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment take part in an artillery exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment pass out care packages and Christmas stockings to fellow soldiers and workers on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment pass out care packages and Christmas stockings to fellow soldiers and workers on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldier from the 3rd Cavalry Regiment take part in a Christmas Eve celebration with soldiers from the Polish army's 21st Mountain Brigade on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment take part in a Christmas Eve celebration with soldiers from the Polish army's 21st Mountain Brigade on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

A U.S. soldier from the 3rd Cavalry Regiment relaxes in his quarters after taking part in a mortar exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

Brigadier General Christopher Bentley (2nd R) and U.S. soldiers from the 3rd Cavalry Regiment take part in a Christmas Eve celebration with soldiers from the Polish army's 21st Mountain Brigade on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment fire a 120mm mortar during an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment take part in a Christmas Eve celebration with soldiers from the Polish army's 21st Mountain Brigade on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment fire a 120mm mortar during an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment unpack ordinance to replace mortar rounds fired during an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

A U.S. soldier from the 3rd Cavalry Regiment relaxes in his quarters after taking part in a mortar exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment fire a 120mm mortar during an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment fire a 120mm mortar during an exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment prepare for an artillery exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment pass out care packages and Christmas stockings to fellow soldiers and workers on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. soldiers from the 3rd Cavalry Regiment take part in an artillery exercise on forward operating base Gamberi in the Laghman province of Afghanistan December 24, 2014. (Photo by Lucas Jackson/Reuters)

U.S. troops from the Nato-led International Security Assistance Force sings Christmas carols during a Christmas Eve celebrations at Bagram Airfield, north of Kabul, December 24, 2014. (Photo by Mohammad Ismail/Reuters)

U.S. troops from the Nato-led International Security Assistance Force (ISAF) light candles during Christmas Eve celebrations at Bagram Airfield, north of Kabul, December 24, 2014. (Photo by Mohammad Ismail/Reuters)