In 1861, Lincoln only sought to preserve the Union and the secession was driven by a desire to protect and promote slavery. The evidence is clear on both counts.
Lincoln fought to preserve the REVENUES (aka tariffs). Secession was due to the desire for self-government, as Lincoln was elected without receiving a single Southern vote. The remainder of the seceding states withdrew from the union AFTER Lincoln declared war on the South and demanded troops from each of the remaining states. Those governors correctly believed that the purpose of their militias was not to wage war on sister states.
The evidence is clear on both counts.