Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.

Context: Joshua Lawrence Chamberlain (born Lawrence Joshua Chamberlain, September 8, 1828 -- February 24, 1914) was an American college professor from the State of Maine, who volunteered during the American Civil War to join the Union Army.
Question: To which branch did Joshua Chamberlain belong?
union army