Countries like Mexico, Cuba, and Brazil joined the war shortly after the United States did. Did they contribute a lot the allied cause or was their declaration of war more symbolic?
More can always be said, but /u/Bacarruda wrote about Mexico in this older comment.
Brazil allowed the US to use a base in Natal to fly aircraft between South America and West Africa. They also recruited one hundred thousand workers from the Northeast of Brazil to collect rubber in the Amazon rainforest.