A lot of people forget that the Second World War, by definition, involved a lot more countries than the U.S. and U.K.
Increasingly better-known, but still underappreciated, is the role of the Soviet Union, which took on 90% of Axis forces, dealt the first decisive blow in Stalingrad, and ultimately took the fight to Berlin, ending the war at the cost of 25-27 million citizens — about half of whom were civilians.
China, which is barely acknowledged as a combatant, served a similarly morbid but crucial function: its large population, tenacity, and willingness to be as brutal as the enemy meant that it took up the bulk of Japanese manpower while losing tens of millions of people in the process, including many civilians. Hence why it is one of only five countries with permanent seats on the U.N. Security Council, as acknowledgement of its role as one of the “Big Four” during the war.
Beyond these two juggernauts — whose importance was acknowledged by the Americans at the time — were dozens of other countries and factions who contributed to the Allied cause, often at great sacrifice.
British India (which included Pakistan and Bangladesh) raised the largest volunteer army in history, at over 2.5 million men.
Canada was among the first to join the conflict, raised an army of 1.1 million (out of a population of just 11 million), had the second toughest landing on D-Day, broke through enemy lines faster and deeper than anyone else, and almost singlehandedly liberated Belgium and the Netherlands, which were in the midst of famine. It also helped develop modern special forces and contributed to the Manhattan Project.
Greece inflicted the first big defeat in the war when it reversed an Italian invasion, forcing the Germans to divert resources and manpower from the Soviet resistance to crush them (though dogged resistance continued until the end).
Poland, France, and Yugoslavia, though each knocked out early, continued to maintain one of the most sophisticated and effective resistance forces in history. Poland was especially fierce, never formally surrendering to the Axis and creating a full fledged underground state, complete with universities and courts; they also cracked the German Enigma Code. The Yugoslav Partisans were among the most feared and resilient fighters in the war.
British-trained Czech and Slovak commandos assassinated Reinhard Heydrich, one of the highest ranking and most feared Nazis. Norwegians sabotaged a vital facility to prevent the German’s nuclear weapons program. The Dutch saved more Jews per capita than any other nation, while the Albanians were the only people in Europe to have more Jews in their country after the war than before, due to their steadfast tradition of providing sanctuary to neighbors and even strangers. Australia — which raised an army of 1 million out of a population of less than 9 million — stood alone in Asia for years and was a key ally in the Pacific. Brazil joined in the invasion of Italy, warranting the Axis to produce Portuguese-language propaganda leaflets.
So though the U.S. of course played one of the biggest and most important roles, both militarily and financially, there were a lot of others who fought, died, and did remarkable things in the global conflict.