The American Civil War had raged for several months, and the northern nation needed a commander for its armies. General Winfield Scott was capable, but far too old to keep command over what he understood would be a years-long war. Irvin McDowell's defeat at Bull Run showed that he was incapable. Many believed George McClellan, commander of the Department of Ohio, would be given the command, but his plans about an invasion of Virginia from the west and a campaign along the Ohio River were the source of much derision. Not even the political squawking of his acquaintance Salmon P. Chase could push him for general.
General Frémont would piece together his Union Army for a fast invasion of Virginia. Instead of waiting for spring, Frémont set out in September of 1861. He would prove ruthless against the rebels, as many of his policies in Missouri had shown. Warnings came from General Henry Wagner Halleck (Frémont's replacement in the west) of the mess the general left behind, but these were ignored as simple backroom military gossip.
Frémont's campaign would be an initial success out of his impetuousness. General Joseph E. Johnston, commander of the Southern Army of the Potomac, would fall back, drawing Frémont deeper into Southern territory, where his armies would wreak havoc. Newspapers from both the North and, especially, the South decried the horrors of war. Finally, only five miles from Richmond, at the Battle of Seven Pines (or of Fair Oaks to the Union) on November 30, 1861, Johnston would counterattack. The battle was bloody and inconclusive, except that Johnston had stopped the approach of Frémont.
Rather than beginning a siege for the winter, Frémont regrouped for another attack and assaulted the Richmond defenses. A more stalwart general may have waited, but Frémont meant to end the war and end slavery. Just as Confederate President Jefferson Davis pulled Johnston from command and replaced him with his adviser Robert E. Lee, Frémont threw his soldiers against the defensive works. It was a gamble that could have won the Civil War for the Union.
Instead, the attack proved to be a disaster. Dead piled up as Frémont's troops were unable to crack the defenses. Lee held the city with everything he could scrounge, and reinforcements poured in from all over Virginia in the Southern counterattack, most notably General Jackson and his Stonewall Brigade as well as the cavalry of JEB. Finally, on December 3, Frémont would die from wounds incurred the day before and the remnants of the Union forces would retreat to Fort Monroe, where they had landed months before.
Despite the terrible setback, Lincoln would refuse to allow the South to secede. The retired Winfield Scott's “Anaconda Plan” called for the conquest of the Mississippi, which would be initially a tactical success in 1862. But, the bloodthirsty General Ulysses S. Grant would prove the undoing of the United States as his losses during battles proved unacceptable. Numerous battles began to turn potential victories into defeats from shortages of men and materiel supplied. With logistics failing in both the western and eastern theaters, Union troops would fail to catch up with Lee's Army of the North before it took Harrisburg, PA. The Southern victory would cut off Maryland and Washington D.C. from the rest of the Union. The subsequent revolt in Maryland would cause another wave of secession, and European nations would begin to recognize diplomatically the Confederate States of America.
George McClellan would win narrowly the election of 1864, ousting Lincoln. While the war would drag on for another three years, eventually McClellan would organize the Treaty of Washington of 1868. Peace would settle over America for a time until border disputes in the coming decades again caused friction between the two nations.
In reality, McClellan, not Frémont, was named general-in-chief. Though McClellan is legendary in history for his “cowardice” on the battlefield (depending too much on lackluster intelligence from Pinkerton scouts and spies), it was his understanding of logistics that arguably won the war for the North. While tepid in battle, McClellan was able to use 1861-62 to organize his command, train troops, and provide for supply-lines. The Union was not ready for a long war, but McClellan's patience and distrust of fighting gave it the resources needed for victory.