ON THE OCCASION of the sesquicentennial of the start of the Civil War, the National Archives delves back 150 years to shed some modern light on the most divisive period in American history.

Discovering the Civil War” is not about muskets in glass cases and decaying camp beds — this lively, interactive exhibition seeks to snap the conflict into focus through its mysteries, secrets and twists.

Letters, diaries, maps, photographs, pamphlets receipts, patents and petitions go on display alongside ciphers and cryptograms, conjuring vivid ghosts. We hear from angry seamstresses, see the document with which Virginia seceded, witness the shame of segregated Union forces and shake our heads at the gall of Pvt. Henry Scott, whom it pleased to fight on both sides of the war.

The exhibition is designed holistically, with themes that resonate throughout the period in question — “Spies and Conspiracies” and “Emancipation,” for example — rather than chronologically.

This deep-drilling show unfolds in two parts: “The Beginnings” runs until Sept. 6; “Consequences” opens on Nov. 20 and runs through April 17, 2011.

» National Archives, Constitution Avenue between 7th & 9th streets NW; free; 202-633-1000.

Photo courtesy National Archives