The shell from the Confederate mortar, its red fuse glowing “like the wings of a firefly,’’ according to one observer, hung briefly before beginning its descent and exploding directly over Fort Sumter in Charleston harbor.

That first shot, at 4:30 a.m. on April 12, 1861, ignited the greatest, most decisive war in American history. By the time the guns fell silent four years later, slavery was abolished, the national union was preserved and a staggering 620,000 men had died.

The Civil War left an indelible mark on America’s soul. Its pivotal place in the nation’s history is beyond dispute.

“Modern America as we know it was born in 1865,’’ said James I. “Bud” Robertson Jr., one of the country’s most esteemed historians of a conflict that remains enveloped in myth and misunderstanding, not just in the defeated South, but in the North, even after 150 years and the passage of several generations.

Connecticut — where the outbreak of the war will be commemorated by the firing of a ceremonial cannon Tuesday firing April 12 on the north lawn of the state Capitol — is no exception.

The contribution the state made to the Union’s victory was immeasurable. About 55,000 men, 12 percent of the state’s population, served in the war and 5,354 of them perished.

Connecticut factories and shipyards supplied the Union’s armies and navy with huge quantities of guns, ammunition and materiel, while the state’s wives, sisters and mothers took the lead in the care and provisioning of its troops.

More than 130 war monuments and memorials across the state attest to the wartime sacrifice and dedication of Connecticut citizens.

It’s a stirring narrative, to be sure, but one that glosses over some rather unpleasant realities.

Striking Contradictions

While Connecticut was staunchly pro-Union and its residents largely opposed to the spread of slavery, it was also virulently anti-black — the “Georgia of New England,’’ in the words of Massachusetts abolitionist William Lloyd Garrison — and home to an active, vociferous peace movement that came perilously close in the spring of 1863 to toppling the Republican state administration.

In 1864, fueled by a string of Union successes, President Abraham Lincoln was re-elected in a landslide, yet squeaked by in Connecticut, his 2,405-vote margin of victory secured by a change in the state’s constitution that extended voting rights to soldiers serving in the field.

In October 1865, just months after the guns had been stilled, Connecticut voters soundly rejected a state constitutional amendment that would have given blacks the right to vote.

These striking contradictions about how and why Connecticut fought the war and their legacy are examined in “Connecticut in the American Civil War: Slavery, Sacrifice & Survival” (Wesleyan University Press, 2011), the first in-depth look at the state’s Civil War experience published in 46 years.

The book’s author, Matthew Warshauer, a history professor at Central Connecticut State University, said residents today would be surprised to learn of “the intense anti-black racism that existed and that Connecticut didn’t line up” and fully support the war effort.

A co-chairman of the Connecticut Civil War Commemoration Commission, Warshauer, like other historians of the period, believes that the war’s 150th anniversary provides an opportunity for a fresh, more balanced and nuanced look at the conflict. It’s a chance, he said, to examine what it resolved — the end of slavery and the claimed right of state secession — and what it did not — racial and political equality and the limits of federal power, topics that are still hotly contested today.

History is about understanding the themes that extend across generations,’’ Warshauer said.

“And The War Came’’

On April 15, 1861, in response to the attack on Fort Sumter, Lincoln asked for 75,000 troops to help put down the rebellion.

Connecticut’s governor, William A. Buckingham, a Republican, immediately called for volunteers. Within days, the 1st Regiment of the Connecticut Volunteer Infantry was formed in Hartford and the 2nd Connecticut Volunteer Infantry Regiment was mustered in New Haven a few days later.