Americans this weekend mark the 150th anniversary of the first major battle in their bitter 1861-1865 civil war, an engagement that both sides thought would result in a quick victory but instead shocked the public with its high casualty rate. Historians have described the July 21, 1861 Confederate victory as the end of American innocence, when the young nation faced a war that threatened its very existence. War broke out in April 1861 soon after 11 southern states formed the Confederate States of America. The agricultural south relied heavily on slaves to work their rich cotton plantations and feared the new US president, Abraham Lincoln, would eventually set them free. How to deal with slavery had been an unsolved problem since the start of the nation -- the 1776 Declaration of Independence after all claimed that \"all men are created equal\" -- and reached a boiling point with Lincoln\'s election. Lincoln declared the split a \"rebellion\", and in April began to form a large army to preserve the Union. Fearing other states could also leave, Lincoln did not declare an end to slavery until 1863, well into the war. The southerners were certain they could quickly capture Washington, while northerners believed they could easily take the Confederate capital of Richmond, a mere 100 miles (160 kilometers) to the south in Virginia. \"It was America\'s baptism by fire, where naive, romantic ideals of warfare came to an abrupt end,\" said Henry Elliott, a historian with the National Park Service, on a recent tour of the Manassas battlefield. \"The battle ended any notion that the war would be quickly decided.\" Enthusiastic, ill-trained soldiers rushed to the US capital wearing flashy uniforms often supplied by their states. Some wore Scottish kilts, while others dressed like French Zouave soldiers. Gray and blue uniforms abounded. The pressure for quick action was enormous. \"The Nation\'s War-Cry! Forward to Richmond!\" screamed the New York Tribune in a June headline. \"Why is Richmond not taken? Why is not our army moving southward ... scattering the hosts of treason before it like dead leaves?\" added the Freedom\'s Champion newspaper from Atchison, Kansas in mid-July. Union commander Irvin McDowell warned his untested soldiers were not ready. \"You are green, it is true,\" Lincoln reportedly told him. \"But they are also green. You agree green alike.\" In July a Union army of 30,000 finally headed west from Washington aiming to capture a key railroad junction in Manassas, and then move on Richmond. A slightly smaller Confederate force lined up at Bull Run, a shallow creek with steep banks ideal for defense, to stop them. McDowell\'s plan was to feint an attack on the main Confederate line, then swing his troops around their lines in a surprise maneuver. Union soldiers however got lost in a long pre-dawn march early July 21, and by the time they were ready to attack the Confederates were waiting. \"It was a whirlwind of bullets,\" a Confederate survivor later wrote. \"Our men fell constantly. The deadly missives rained like hail among the boughs and trees.\" The cannonade and gunfire was so intense that Sunday churchgoers in downtown Washington, some 30 miles (48 kilometers) to the east, mistook the noise for thunder. In the days before smokeless gunpowder, battles were loud, confusing affairs, and battlefields were often shrouded in acrid black smoke, leading even the best trained soldiers astray. The outnumbered southerners resisted, but by midday on July 21 had taken heavy casualties and were forced to fall back on a hill. \"Victory! The day is ours!\" cried out McDowell as he visited advancing troops, who cheered wildly at seeing their commander. The exhausted Union soldiers paused to wait for rear units to arrive for a final attack. This gave news reporters time to rush off to file stories. \"We have carried the day. The rebels... are totally routed,\" the New York Herald reporter telegraphed his editors. In New York, where competition among the city\'s 18 dailies was ferocious, the late editions announced a great Union victory that same afternoon. A cluster of civilians, including US senators and members of Congress, had gathered on a nearby hill and had a picnic as they watched the battle. \"The spectators were all excited, and a lady with an opera glass who was near me was quite beside herself when an unusually heavy discharge roused the current of her blood,\" wrote London Times correspondent William Howard Russell. The afternoon clash was ferocious. A New York soldier wrote that the bullets \"came crashing through the cornfield, singing and whistling around our ears, making the air blue and sulfurous with smoke.\" After hours of intense combat, fresh southern reinforcements arrived. The Union soldiers wavered, panicked and broke, fleeing in disorder. On their rush back to Washington they got caught up with the civilians also rushing home. In the evening, when a New York Times reporter tried to file an update to his earlier \"victory\" story, the military censor closed the telegraph office. Protests broke out in northern cities when the extent of the defeat was finally learned. There were 5,000 casualties that day, an astonishing high rate for the period -- yet small compared to the 23,000 killed one year later in a second battle also fought at Manassas. By the time the war ended in 1865 more than 600,000 soldiers had been killed. \"I had a dim notion about the \'romance\' of a soldier\'s life. I have bravely got over it since,\" a Union soldier who fought at Manassas wrote in a letter home.