I picked the British because no country accomplished so much with so few.
In World War I, the BEF got mauled in the Schlieffen Plan but delayed the German advance enough times (wasn't even that many) for the French to win the Marne and were primarily responsible for blockading Germany and starving it into surrender.
In World War II, the British were along against Germany for nine months and were the first to successfully defend themselves against German aggression. Britiain defeated the Japanese in Burma and the Germans in Africa and it wasn't until the invasion of France where American troops completely outnumbered British in the frontlines against Germany.