Slavery was THE issue?
"I have no purpose, directly or indirectly, to interfere with the institution of slavery in the states where it exists. I believe I have no lawful right to do so."
From Lincoln's first inaugural address
"My paramount object in this struggle is to save the Union, and it is not either to save or destroy slavery."
Lincoln in a 1862 letter to Horace Greeley, New York Daily Tribune editor
A precursor for a War Between the States came in 1832, when South Carolina called a convention to nullify tariff acts of 1828 and 1832, referred to as the "Tariffs of Abominations." A compromise lowering the tariff was reached, averting secession and possibly war. The North favored protective tariffs for their manufacturing industry. The South, which exported agricultural products to and imported manufactured goods from Europe, favored free trade and was hurt by the tariffs. Plus, a northern-dominated Congress enacted laws similar to Britain's Navigation Acts to protect northern shipping interests.
Shortly after Lincoln's election, Congress passed the highly protectionist Morrill tariffs.
That's when the South seceded, setting up a new government. Their constitution was nearly identical to the US. Constitution except that it outlawed protectionist tariffs, business handouts and mandated a two-thirds majority vote for all spending measures.
BTW, 620,000 is the accepted number for WBTS killed, not 900,000.
Most died from disease in any case. I have seen 620,000 as the number of Union dead. Southern dead were a smaller number.
The seccession Must not have been about Lincoln, because it happened before he took office.
I do thank you for the correction.
I confused "Casualties" with "Deaths".
620,000 is a good estimate for "Deaths", but in the campaign from Wilderness to Cold Harbor, the South no longer collected casualty numbers. That hinders accurate study. The Southern commanders also didn't collect much data on the number of their soldiers that they murdered themselves to keep them from returning to their homes. That difference in record keeping helps perpetuate the myth of loyal southern units.