Final answer:
Abraham Lincoln became the president during the Civil War. His opposition to the expansion of slavery contributed to the conflict.
Step-by-step explanation:
The president who became the president during the Civil War is Abraham Lincoln. He was the 16th president of the United States and served from March 1861 until his assassination in April 1865. Lincoln's opposition to the expansion of slavery into new territories was a major factor that led to the secession of southern states and the start of the Civil War.
Learn more about Abraham Lincoln's presidency