The statement that best explains why Russia left World War I before it ended is: The damage of the war caused an internal revolution.
Russia faced significant military defeats and immense social and economic strain due to the war, which ultimately contributed to the February Revolution in 1917. This internal turmoil led to the abdication of Tsar Nicholas II and the establishment of a new government, which eventually sought to withdraw from the war, resulting in the signing of the Treaty of Brest-Litovsk in March 1918.