The American military base in Poland is very important for the USA, but not necessary at all for the Polish interests. Poland finances 100% of the costs of this base – it means that the Polish state pays 100% of the costs to achieve US strategic goals in the region. Poland helps the USA to play its geopolitical game increasing the pressure at Russia.
In 1939, the “ideal scenario” assumed by the UK authorities at the beginning of the war between Germany and the USSR was the partition of Poland, which was sacrificed by the UK to inspire a conflict between Stalin and Hitler. In this situation, the UK would have been able to present itself as a peace-making force, increasing its influence in Europe. Although, this “UK schedule of war” was thwarted by Hitler, who forced Stalin to sign the Molotov-Ribbentrop Pact and attacked France instead of the USSR.
This message is part of the Kremlin’s policy of historical revisionism and an attempt to portray Russia's role in the World War II as not aggressive - see other examples referring to Ribbentrop-Molotov pact here, here and here. The accusations that the UK had “expansionist plans” in Europe and attempted to provoke a war between Germany and the USSR is clear historical manipulation and these events are not confirmed by historical documents and historians.
In terms of the Molotov-Ribbentrop Pact, it is a proven historical fact that it contained the Secret Supplementary Protocol, which assumed the division of Poland and other Eastern European countries between the USSR and Germany. Thus, the signing of the Molotov-Ribbentrop Pact directly caused the German and Soviet military aggression against Poland in September 1939, which resulted in complete occupation of this country by Germany and USSR. See similar examples of Russian historical revisionism concerning Poland here and here.