Short answers, pulled from the story.
Paul von Hindenburg retired from the military in 1911 to make way for younger men. He spent the previous three years hunting game and managing his estate before being summoned to save Germany on the 23rd of August 1914.
The Battle of Tannenberg resulted in the destruction of the Russian Second Army with 92,000 Russians captured and only 14,000 German casualties. This victory transformed Paul von Hindenburg from a forgotten retiree into a national hero and the center of a personality cult.
Paul von Hindenburg and Erich Ludendorff established a de facto military dictatorship known as the Third OHL which dominated Germany for the remainder of the war. They issued a Textbook of Defensive Warfare and implemented the Hindenburg Program to set ambitious benchmarks for arms production.
Paul von Hindenburg died on the 2nd of August 1934 after serving as President of Germany from 1925 to 1934. Following his death Adolf Hitler combined the presidency with the chancellery before declaring himself Führer of Germany.
Paul von Hindenburg appointed Adolf Hitler as chancellor in January 1933 because he believed that the Nazi Party could be controlled within a coalition government. He felt that the only way to save Germany from chaos caused by the Great Depression was to bring the Nazis into the government.