1st New Hampshire Infantry Regiment filled its ranks within two weeks of President Lincoln's call for 70,000 men on April 15, 1861. Between April 17 and 30, 1861, not less than 2,004 men volunteered to fight for the Union in the American Civil War. The volunteers organized and mustered at "Camp Union," the Fair Grounds of the Merrimack County Agricultural Society on the east side of the Merrimack River, in Concord between May 1 and May 7, 1861. After the 1st NH was filled, volunteers were given their choice to enlist in the 2nd New Hampshire Volunteer Regiment or serve their three months as the garrison of Fort Constitution at Portsmouth Harbor. Four hundred and ninety-six (496) enlisted in the 2nd NH, and the remainder were sent to Fort Constitution.