” The North’s invasion of the South was controversial at the time and remains so today.
Many people believe Lincoln “fought to free the slaves” and to overthrow the pro-slavery United States Constitution. “
Agreed.
President Lincoln’s external position was maybe subtly ambiguous:
“If I could save the Union without freeing any slave I would do it, and if I could save it by freeing all the slaves I would do it, and if I could save it by freeing some and leaving others alone I would also do that.”
If Lincoln actually did that it is a real problem. Because a U.S. president is not supposed to use the military to violently overthrow the United States Constitution.
The South had feared Lincoln would do just that if he was elected. A president is supposed to advocate peaceful changes to the Constitution when change is needed using the well-thought out amendment process.
The abolition of slavery for example could have been proposed and debated and voted on in 1861, or earlier, and all the killings and destruction avoided.
No member of Congress proposed an amendment to abolish slavery before the war - not even Lincoln when he served in Congress.