Churchill did declare war on Finland to please Stalin. This is the only time that two democracies have been at war. Russia invaded Finland at the same time that the Nazis invaded Norway. The Fins were a lot more successful at repelling the Russians than the Norwegians had been at fighting the Nazis. So when the Nazis attacked Russia the Fins found themselves on the same side as the Nazis. Later on in the war the allies were able to forge a seperate peace with Finland, and get the Russians focussed on the Nazis.
We'd just fought the battle of Britain alone, and held out for a year. America didn't look like they were coming to our aid and the American ambassador Joe Kennedy had pretty much written us off. Russia was a valuable ally against Hitler, and the only ally we had. Stalin and Churchill formed a deep understanding. Churchill knew of Stalin's failings, but he thought he could influence him. In the UK at this time Stalin was popularly known as Uncle Jo. This was all to do with the wartime necessity of keeping up morale and good relations with our ally.
Churchill did know what Stalin was like, but he had a lot of problems convincing Roosevelt, and almost no chance of convincing Truman. Truman found out for himself a bit late in the day.