Short answers, pulled from the story.
The United Kingdom, France, and Russia signed an agreement in 1907 to form the Triple Entente. This diplomatic realignment created a counterweight to the Triple Alliance of Germany, Austria-Hungary, and Italy.
Japan joined the Entente by declaring war on Germany on the 23rd of August 1914. Japanese forces surrounded the German Treaty Port of Tsingtao on the 2nd of September 1914, which surrendered on the 7th of November.
Total casualties amounted to 18,744,547, representing 49% of all mobilized forces. The Allied Powers mobilized a total of 42,244,409 personnel with military fatalities reaching 5,741,389 across all participating nations.
Douglas Haig commanded the British Expeditionary Force from December 1915 until November 1918. Marshal Ferdinand Foch became Supreme Allied Commander on the 26th of March 1918.
Serbia suffered the greatest proportional losses of any combatant, with over 25% of those mobilized becoming casualties. Including civilians and deaths from disease, over 1.2 million died in Serbia, nearly 30% of the entire population.