Why would the Soviets attack American troops in Warsaw or anywhere else?
Soviets already executed Allied soldiers belonging to Polish government-not only in Katyn but also those fighting in Vilnius, Lvov and Warsaw Uprisings(often after they cooperated with Soviets forces)
You assume that the aim of the USSR was to occupy as much of Eastern Europe as possible at any cost. They were able to occupy the territory they did because they were in pursuit of the Germans. Like the western allies they wanted governments in those areas they occupied to be sympathetic to their political system. Their purpose was not to conquer territory for its own sake. This is simply cold war propaganda served up as history.
Nope this are facts. USSR wanted to regain territories belonging to former Tsar Empire since 1920s:
http://en.wikipedia.org/wiki/Soviet-German_cooperation
On July 19, 1920, Kopp told the German Foreign Office that Russia wanted "a common frontier with Germany, south of Lithuania, approximately on a line with Bialystok". In other words, Poland was to disappear completely. These promptings were repeated over the years, with the Russians always anxious to stress that ideological differences between the two governments were of no account; all that mattered was that the two countries were pursuing the same foreign policy objectives.
On Thursday, April 15, 1920, Victor Kopp, Soviet Russia's special representative sent by Lenin to Berlin, asked at the German Foreign Office whether "there was any possibility of combining the German and the Red Army for a joint war on Poland".
Stalin was left out of the general settlement of western europe and so made life difficult for allied observers in Poland and other eastern areas
Stalin also executed hundreds of thousands of Poles, and ordered murder of Polish Home Army members, Soviet troops opened fire on Allied planes droping supplies to Warsaw during Warsaw Uprising.
Lets not forget also that SU was a vital ally of Germany since 1920 to 1941, rebuilded German army, provided it with training facilites and NKVD and Gestapo had a joint training center in Zakopane Poland to coordinate fighting Polish resistence movement in occupied territories.
http://www.yale.edu/lawweb/avalon/nazsov/sesupp1.htm
After the Government of the German Reich and the Government of the U.S.S.R. have, by means of the treaty signed today, definitively settled the problems arising from the collapse of the Polish state and have thereby created a sure foundation for a lasting peace in Eastern Europe, they mutually express their conviction that it would serve the true interest of all peoples to put an end to the state of war existing a present between Germany on the one side and England and France on the other. Both Governments will therefore direct their common efforts, jointly with other friendly powers if occasion arises, toward attaining this goal as soon as possible.
Should, however, the efforts of the two Governments remain fruitless, this would demonstrate the fact that England and France are responsible for the continuation of the war, whereupon, in case of the continuation of the war, the Governments of Germany and of the U.S.S.R. shall engage in mutual consultations with regard to necessary measures.
Moscow, September 28,1939.
For the Government of the German Reich:
J. RIBBENTROP
By authority of the Government of the U.S.S.R.:
V. MOLOTOV
http://www.yale.edu/lawweb/avalon/nazsov/sesupp1.htm
The undersigned plenipotentiaries, on concluding the German Russian Boundary and Friendship Treaty, have declared their agreement upon the following:
Both parties will tolerate in their territories no Polish agitation which affects the territories of the other party. They will suppress in their territories all beginnings of such agitation and inform each other concerning suitable measures for this purpose.
Moscow, September 28,1939.
For the Government of the German Retch:
J. RIBBENTROP
By authority of the Government of the U.S.S.R.:
W. MOLOTOV
In 1939 Soviets and Nazis held joint victory parade after their invasion of Poland :
http://brama.bereza.by.ru/nomer21/defilade.jpg
http://www.interet-general.info/IMG/brestlitovsk1939.jpg
http://www.ska.pl/biorytm/tankista.jpg
The reasons for fighting Soviets were there, but sadly they were never pursued sentecing milions of people soviet enslavement and exploitation.
So FDR would do what if the Soviets started shooting? Surrender? Sue for peace?
At least two war plans were made by Allies of engaging Soviet regime.
http://www.history.neu.edu/PRO2/
http://members.tripod.com/~american_almanac/church.htm
The other was made early in the war and involved bombing Baku oilfields-during the time Soviets provided German war machine with crucial resources enabling them to wage war on western countries.
http://www.themilitarybookreview.com/html/OperationPike.shtml