If The Soviet Union and Nazi Germany opposed eachother in WW2, why did they both invade poland?

Your claim isn't true. It's also almost skewed 180 degrees. Because wasn't it American CONSERVATIVES who were the most opposed to any US involvement in fighting Hitler?

Right up until Pearl Harbor and the pro-war furor it caused? True: The American Communist Party did flip-flop on whether to fight against Hitler -- twice, as I'll explain below. But American "liberalism" and "Progressivism" both date back to before the Bolshevik Revolution, and there were many non-Communist liberals in the USA in the 1930s.

Many of them did think Hitler needed to be stopped. But the Communists did waffle on Hitler. Why?

For Communist parties around the world, the Soviet Union under Stalin -- however horrible his crimes -- represented the "vanguard" of world revolution and needed protection. Loyal Communists therefore supported whatever Soviet foreign policy was at the moment, and on the question of Hitler, Soviet policy shifted. Ex-Communist Arthur Koestler, in "The God That Failed" notes that at one point in Germany, the Communist Party thought Hitler was somewhat harmless to their goals, and that German social democrats were their main foe.

They thus focused on fighting the social democrats -- until Hitler took power and began arresting Communists. Then their line shifted. From 1933 through 1938, Communist parties around the world were strongly anti-Nazi and anti-Fascist, and under the "Popular Front" policy, they tried to make common cause with democratic socialists, liberals and other progressives they had formerly despised.

Especially during the Spanish Civil War, when Hitler & Mussolini backed the rightwing forces of Gen. Franco in trying to overthrow a democratically elected Spanish Republic, Communists came to Spain from other countries -- including the US -- to fight for the Republic and against the Germans, Italian fascists and Franco forces. Communists in the US and elsewhere also tried to enlist the Western democracies on the side of the Spanish Republic and against the fascists and nazis, but in the US, England and France, anti-Communist governments refused to do this.

Stalin himself, through his Jewish foreign minister Litvinov, still continued trying to rally the French & British to stop Hitler right up until the Munich conference of 1938. But when Stalin saw that the West would rather allow Red Russia and Nazi Germany to fight each other, rather than joining with Russia to stop Germany, Stalin then flipped positions -- naming a new, non-Jewish foreign minister, Molotov, who negotiated the notorious Hitler - Stalin pact of "non-aggression." With the Hitler-Stalin pact, most American Communists instantly changed their line and opposed Western military opposition to Hitler -- in order to support Soviet foreign policy.

The American CP, anyway, lost huge numbers of supporters because of this, especially among Jews. But then when Hitler invaded Russia, the CP switched tacks and became loudly anti-Nazi again -- since the USSR now was under attack. All this flip-flopping cost the CP dearly, as leftwing historian Christopher Lasch reports in "The Agony of the American Left."

But many "liberals" in the 1930s were NOT Communists, and they didn't all flip-flop on Hitler as the CP did. President Franklin Roosevelt, the greatest liberal politician the US has had, played a different game. At least some biographers report that he refused to take a military stand against Hitler before 1938 because he hoped that Hitler & Stalin would destroy each other.

He didn't want to get in Hitler's way when it came to fighting the Reds. Roosevelt also stayed out of World War II at first to placate a strong American "isolationist" or "Fortress America," movement, some of it politically conservative, that rejected new foreign wars because of widespread disillusionment with the results of WW I. Unwilling to provoke the isolationists, but increasingly anti-Nazi because of Hitler's aggressions, Roosevelt quietly pushed for "lend lease" arrangements to help Britain fight the Nazis between 1939 and 1941.

Then he changed following the Pearl Harbor attack, when a huge swing of US public opinion made it easy to rally support for war. -- democratic socialist "only the truth is revolutionary.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.