Just a brief timeline to figure out the background to the RP.
1908, Tunguska event.
1914-18, The Great War, the US does not join in the conflict.
1917, Russian Revolution is crushed leaving Russia under Tsarist rule.
1921, Russia initiated a communications blackout with the rest of the world, and built a wall against its European border called the "Red Curtain". Word spreads of small towns, villages, and eventually cities in Russia and Eastern Europe that are destroyed within a single night. A strange cold front also develops over Russia and spreads outwards. European intelligence agencies attempt to listen in on Russian radio stations, only to hear a single message repeated over and over: "Brotherhood, Strength, and Fortitude...in the face of the angry night.
1922-1949, the 'Cold War'. Weary of an invasion by an isolationist Russian Empire, a prosperous European alliance is formed thus helping to avoid the Great Depression and the rise of the Nazis in Germany, and World War II. European military and Intell units watch the Red Curtain, teams sent into Russia disappear and fail to return. The British Empire reviews and strengthens its defences round India and in the Far East, focused on the land approaches.
December 1949, the Red Curtain is suddenly breached by numerous Chimeran forces. The Chimera overrun continental Europe by February 1950, leaving very few survivors. The Chimera then proceed to dig underneath the English Channel, invading Britain in late 1950. The bulk of the British forces are wiped out, with the rest fleeing to scattered bases in the northern parts of England. The RAF manages to take the Royal Family and surviving members of Parliment to safety, first in Canada and then Australia.
Torchwood is placed under the direct command of the British Intelligence Corps along with the Royal Marines Commando, effectively becoming the British Resistance group. Torchwood assists with the capture of a Chimeran 'Angel' sometime in late 1950/early 1951, but with great loss of life.
1951, in a deal brokered between Captain Rachel Parker (British Intel and head of the Resistance) and the US Government (who've told the American people that the Chimeran invasion of Europe was another flu epidemic) an American liberation force is sent to Britian in exchange of a 'secert weapon' (the Angel). Operation Morning Star begins in June 1951, but the American forces are ambushed and largely wiped out in York whilst the British convoy is ambushed enroute through Manchester.