Answer:
WWI -- Russia. Russia entered the first world war with the largest army in the world, standing at 1,400,000 soldiers; when fully mobilized the Russian army expanded to over 5,000,000 soldiers (though at the outset of war Russia could not arm all its soldiers, having a supply of 4.6 million rifles).
Step-by-step explanation:
Russia entered World War I in the three days succeeding July 28, 1914 — beginning with Austria-Hungary's declaration of war against Serbia, a Russian ally. The threat to France caused Britain to declare war on Germany on August 4. ... The main belligerents had been established