(Sorry if this is terrible. This would be my first actual TL, so let me know about any comments/suggestions/etc.)
1860: Abraham Lincoln is elected President of the United States, amongst staunch opposition in the Southern States. Under a secessionist movement spearheaded by the charismatic southern politician Robert E. Lee, 9 Southern States secede from the Union: Virginia, the Carolinas, Georgia, Alabama, Mississippi, Louisiana, Texas and Tennessee. They quickly form the Confederated States of America under provisional President Robert E. Lee's leadership.
1861: The USA, attempting to minimize losses to secession, attempt to permit secession on the term that they maintain federal possessions within the CSA. The Confederate Congress unanimously rejects this suggestion, and the Confederate Army opens fire on supply ships sent to relive the Union forces at Fort Sumter. The Civil War begins, with Maryland, Florida, and Missouri quickly voting for secession, and Detroit and Kentucky declaring themselves neutral parties. Despite Maryland's voting for secession, the federal army quickly spreads from the capital, and secures the state for the Union, amidst civil, armed, and international protest. Robert E. Lee successfully dispatches an ambassador to the British Empire, though his attempts at a French counterpart are interrupted by an hastily established Naval blockade of the CSA. The Battle of Shiloh ends with Union forces routed by Confederate Commander Thomas "Stonewall" Jackson, who is the clear decision for Commander of the Confederate Army.
1862: The British Empire acknowledges the CSA as an independent nation, and immediately demands trade rights, which are granted by the US when Royal Navy ships are spotted heading south from Hudson Bay, and the naval blockade of the Confederacy is broken. Confederate forces under Stonewall Jackson's command launch a two pronged invasion of Maryland, surrounding and crushing Gen. Meade's Army of the Potomac, and beginning the Siege of Washington. The US Navy, despite having declared an end to the blockade, begin raiding Confederate trade ships meant for the British Isles. The British Empire, in response to the raids, begin sinking US Trade ships, and begin massing forces in Canada.
1863: The Siege of Washington ends with the destruction of the Library of Congress by Confederate forces, and the surrender of the federal forces present, as well as the capture of two of Lincoln's cabinet members and over a dozen US representatives. Maryland enters the Confederacy. The federal army seizes control of Kentucky and Detroit. The Royal Navy, in response to an attack by US Naval forces, blockade New York City, and a state of war is declared when US forces cross the US-Canada border.
1864: The USA is forced to sue for peace when a Confederate army from Texas invades and captures Southern California, and Jackson leads successful invasions of Kentucky and Detroit. The terms are seen as 'agreeable' by those wishing to see an end to the war, and 'horrendous' by those against the Treaty of San Jose. The Treaty gives the Confederacy control of all territories once called 'slave territories' by the Missouri Compromise, including Southern California, but returns control of Detroit and Kentucky to the Union. Maryland is declared 'de-militarized' and, though under the CSA's domestic control, is not available for militarization. Washington remains the Union capital. The USA moves forces from its southern front to the stalled northern front, as the British reinforce Canadian troops in Ottawa, Toronto and Montreal, in preemption of US invasion.
1865: The US Army begins a siege of Montreal and Ottawa. The British request an alliance with the CSA, but, still war-weary and relatively unstable, the CSA declines. The CSA opens trade with France, Prussia, Russia, and South America. Mexico disputes the southern border of the CSA along the Rio Grande, and the CSA begins a build up of its naval forces. The British successfully defeat the US in Montreal and Ottawa, but fail to engage Toronto. The British launch a large scale invasion of Maine and Oregon territories. The latter is defeated, and the prior remains a fluid battle for the greater part of a year.
1866: Mexico attempts to drive CSA forces from the Rio Grande to the Rio Nueces, and the Second Mexican-American War begins and ends within 7 months, with the CSA gaining the region of Baja after soundly defeating the Mexican Army at the Battle of Monterrey, and blockading the Gulf. The US Army is glaringly beaten during the Second Battle of Montreal, and the Royal Navy is dealt a crushing blow when 12 Warships are forced into the Gulf of Lawrence and destroyed.
1867: The Confederate government begins greatly limiting the institution of slavery, under the urging of a re-elected Robert E. Lee. The British and the United States agree to a return to antebellum borders after the US Army captures Ottawa, the Royal Navy shells Washington, and riots break out in New York city, causing millions of dollars in damages throughout the year. The American War ends on November 12, 1867, a largely crushing defeat to the Union, two startling victories for the fledgling Confederacy, and a small victory for the British Empire.
That's all I have for now. I may post more depending on the reaction. In a nutshell: the CSA and Britain end up at war with the USA, Mexico, and France, again, and then I may start getting into World War 1, and World War 2.