The phrase 'over by Christmas' is usually attributed to 1914 however it is likely that it predates this: the origin being the Crimea, the British deployment of September 1853 met a subsequent Russian counterattack and made it obvious to British forces it would not be over by Christmas, though the British press continued to insist it would be.
Optimism on this subject is not uniquely British: Napoleon, pictured above, made it to Moscow by September 1812 only to be forced into the worst retreat in military history, all but destroying the Grand Armee. It was over by Christmas but not in the way he had imagined. Hitler in his optimism famously failed to issue his forces with winter clothing as they invaded Russia in 1941, however his panzers ground to a halt in the mud short of Moscow, as his jack booted kick failed to bring the 'whole rotten structure' of Stalin's Russia 'crashing down'.
This Christmas, British forces will be deployed on combat operations as they have been for every Christmas, bar one since 1945. As the War on Terror enters its fifteenth year there have been a few over by Christmases: George W Bush 'Mission Accomplished' from the deck of USS Abraham Lincoln in May 2003 ranks as one and another, for the British, John Reid infamously briefing in Kabul April 2006 that he 'would be perfectly happy' if British troops left Afghanistan without firing a shot.
For the RAF based in the old crusader isle of Cyprus this will not be a busy time. They may fly but after over 8000 sorties prior to their joining the air crusade over Syria there is little left to bomb. Time perhaps to take trip to the Crusader port of Kyrenia and enjoy some winter sun. On the other side of the world British forces are deployed in Afghanistan, though thankfully none of our regular troops are in the siege of Sangin, the SAS probably are, but that will never be confirmed.
Winter sieges are always problematic for the defenders but Sangin is not Stalingrad, with Goering's false reassurances and Goebbel's fake carols from the troops broadcast to the German people in the Christmas 1942. However even modern air power is susceptible to low cloud cover, and reliance on the unreliable ANA could all play their part. The outcome for this town - so talismanic to the British people after our sacrifice there - may be uncertain this Christmas, but it is also irrelevant: Helmand, Kandahar have long been dominated by the Taliban, our final defeat in one town will not change that. If the SAS are there I pray they safely get on the last helicopter out, a luxury that will not be afforded to all those who fight alongside them.
Whatever the outcome we can be assured this war will not be over by Christmas.