The Russian Empire.
The Russian Empire was the only allied belligerent on the Eastern Front. It had to fight forces from both Germany and The Austro-Hungarian Empire although actions against the Ottomans would be supported by the British and the French in the Middle East.
The Russian Empire was still relatively backwards as far as many European countries were concerned. The greater part of their rural population was still part of a feudal system although strides had been made toward a fair society over the 19th century. The 19th century saw Russia devastated by the invasion of Napoleon (something which had affected many of the European countries). It also saw the Crimean war fought against Britain and France. The conflict was a mess for all who fought with the lack of good quality officers in the British army keenly felt. This war was fought on Russian soil though and left Russia in a weak position on the European stage after it lost. It was Russia’s alliances in the Balkans which would be a large catalyst for the escalation of tensions into a full Europe-wide war.
The Russian Empire had been going through a period of political unrest with an unpopular Tsar who had been targeted for assassination. Russia had also been embroiled in a war with Japan in 1905 which had shown that its armed forces needed to be improved (a situation that had happened many years before during the Napoleonic period and before).
The total Russian population was over 125,500,000 although it only mobilised 12,000,000 troops. Part of the reason for this was the vast size of Russia and also due to Russia disastrous defeats which lead a revolution to break out which pulled Russia out of the war in late 1917. Russia may have been a large country with a population that could support a large military but it was the supply of this military that caused issues. There were massive shortages of equipment in the early years of the war which lead to a disgruntled military.
Russia casualties were predictably heavy with 1.7 million killed and nearly 5 million wounded. The figures for missing and prisoners stand at a staggering 2.5 million. This shows that three-quarters of the entire military mobilised were casualties of the war. These numbers are however estimates. The Revolution from 1917 onwards meant that very little paperwork giving precise figures survived the conflict.