Final answer:
The issue that preoccupied thoughts during Lincoln's first Inaugural Address was the secession of Southern states and the subject of slavery, focusing on the permanence of the Union and the unlawfulness of secession.
Step-by-step explanation:
According to Abraham Lincoln in his “Second Inaugural Address”, the issue that occupied people's thoughts and attention at the time of his first Inaugural Address was the impending conflict due to the secession of Southern states and the overarching issue of slavery. In his first address, he attempted to calm fears in the South and maintain the integrity of the Union. Lincoln made clear his commitment to not interfere with slavery in the states where it already existed, but he also highlighted the permanent nature of the Union and the illegality of secession. His plea for reconciliation and unity underscored the tensions of the time.