One of the historian’s most difficult tasks is to convey a sense of mood to a modern audience for whom that mood is inconceivable.

In “The Siege of Washington: The Untold Story of the Twelve Days That Shook the Union,” brothers John and Charles Lockwood make superb use of contemporary accounts to go beyond facts, circumstances and events to capture the feeling of dread hanging over the nation’s capital in the days immediately following the fall of Fort Sumter in April 1861.

The story, despite the subtitle, is not really untold. The broad outlines appear in all comprehensive histories of the war. Historians of wartime Washington have paid particular attention to the period — excellent, if abbreviated, accounts can be found in Margaret Leech’s 1941 classic “Reveille in Washington” and Ernest B. Furgurson’s “Freedom Rising: Washington in the Civil War,” published in 2004.

But the Lockwoods — both born and raised in Washington — present the first full-length treatment, and it is a detail-laden treasure.

They present a day-by-day account, from April 15, the day President Abraham Lincoln’s call for 75,000 volunteers to defend the Union was published, to April 25, the day the 7th New York Regiment arrived in the city, relieving the siege.

That structure lends itself to mounting tension and helps provide a sense of the fear and panic that enveloped the federal capital, a city with Southern sensibilities that was surrounded by slave-holding states.

It seemed obvious that the Confederates across the Potomac would attack and capture the seat of Northern government, which lacked both natural and man-made defenses. In fact, it remains something of a mystery even today why that didn’t happen.

The Lockwoods offer several plausible explanations. For starters, Southern leaders were thinking tactically rather than strategically at the beginning. They felt it was more important to use scarce resources to capture the arsenals at Harpers Ferry and Norfolk, Va.

Northerners also wildly overestimated the number and effectiveness of Southern troops so early in the conflict, and many of the soldiers the South did have were far from Washington.

Lastly, Confederate caution ruled. A dash for Washington that succeeded might well bring a quick end to the war and win independence for the Confederacy. But an attack that bold that failed might have dire consequences for the fledgling government.

So on the Virginia side they stayed, and a cold peace reigned until the First Battle of Manassas three months later.

But the citizens of Washington were not privy to Southern deliberations, and they worried themselves into a frazzle. Here is where the Lockwoods shine: Using newspaper accounts, diaries and letters, they paint a vivid portrait of a city on the edge, its citizens fearful of invasion, suspicious of their neighbors and desperately wondering when help would arrive.

Help was on the way, but it took awhile to get here. State governors responded with enthusiasm to Lincoln’s call for volunteers, and companies were quickly organized. Many had already been formed in anticipation of war.

If the siege could be said to have a turning point, it came April 19, when the 6th Massachusetts Regiment was attacked by a mob in Baltimore.

Slave-holding Maryland was a nest of intrigue, and Baltimore had a well-earned reputation as a violent place. In those days, Charm City was known as Mob City for the annual Election Day riots that occurred in the years leading to civil war.

The immediate effect on Washingtonians of the Baltimore attack was to increase the fear. People began to wonder if anybody was ever going to come to their aid. Secessionists in towns around the capital tried to block railroad access.

But the attack on the 6th Massachusetts — coming as it did on the anniversary of the battles of Lexington and Concord — galvanized the North. Members of the 6th became the “Minute Men of ’61.” Washington, which Lucius Chittenden wrote bore the “aspects of a besieged town,” was still having to fend for itself, but at least now its residents knew there were Northern soldiers moving to its relief.

The April 24 edition of the Washington Star was headlined “NO TRAINS — NO TELEGRAPH — NO ANYTHING,” but it was almost over at that point. The next day, the eagerly awaited 7th New York Regiment marched into town, and most of the fear died away amid concerts and celebrations.

The Civil War is probably the most well-chronicled event in American history. The Lockwoods don’t unearth any startlingly new material here — all their major sources have been used before. But “The Siege of Washington” adds to our sum of knowledge about the war by putting those earliest days on a well-lighted stage and focuses our attention on just the right actors.