When the Nazis and the Soviets invaded Poland in 1939, why did the French and British only declare war on the Germans?
Interestingly enough they did actively consider it, but not over Poland. The British came extremely close to going to war with the Soviets over Finland as they thought it could assist their positions with the Scandinavian neutrals, and possibly even the USA. War Cabinet Archives and the diaries of the Chief of the Imperial General Staff also reveal British policy makers were motivated by a desire to exploit the situation brought about by the invasion of Finland (by the Soviet Union) to achieve a quick end to the war. By utilising pro-Finnish sympathies to legitimise sending allied troops to Finland and Northern Sweden they intended on preventing Germany importing high grade Swedish iron ore which was so vital to her war industry. In addition both public and international opinion played a role. The British sent armaments, planes to the Finns, and even had British volunteers fighting Soviets in Finland.
The plan to sent troops to Finland got the go ahead from Churchill, but had to be called off when the Germans invaded Norway and Denmark. I the German's hadn't invaded Scandinavia then we may well have sent lots of regiments to Finland and ended up at war with the Soviets.
Sources:
Patrick Osborn, Operation Pike: Britain versus the Soviet Union, 1939-1941, (Westport: Greenwood, 2000)
Justin Brooke, The Volunteers: The full story of the British Volunteers in Finland 1939-41, (Worcester: Billing & Sons Ltd, 1990),
These are all great comments and very accurate, but you can answer this question more directly and succinctly: 1) because it wasn't 100% clear that Russia was an enemy quite yet (the enemy of my enemy is my friend, remember? The Nonagression Pact was basically the most off-the-wall thing anyone had ever imagined. Everyone knew/though/suspected that Germany and Russia would fight); 2) because how? Britain was in absolutely no shape to confront a country like Russia. America had no reason to do so at this point, and 3) everyone was fixated on that weirdo Hitler and his next move.
I'll cite sources in the morning if that's necessary.