 Following the First Battle of Bull Run on July 21 , 1861 ( the first major engagement of the war and a disastrous defeat for the untested Union army ) , citizens of the northern states began to realize that the Civil War would not end quickly . Additional troops would be needed beyond the 75 @,@ 000 volunteers that had been called out for 90 days of service . As a result , over the summer of 1861 , volunteers rushed to enlist for a term of three years . The 21st Massachusetts was among the " three year regiments " organized after the First Battle of Bull Run . 
