Lincoln didn’t declare war. The rebels did that, pretending to the independent identity that they legally didn’t have. Then the rebels made illegal war. Then they lost their war.
Then they filed suit asserting that what they had done was legal. They lost that too.
They and their apologists have lied about it ever since.
His intent to abolish slavery was an act of war. You yankee apologist don't realize that abolition translated into economic terror for the south. Such an act would have thrown the southern economy into depression of unparalleled proportion.
A good analogy, would be if some southern environmentalist in 1860 forced every manufactuer type industry in the north to close. How would have that have gone over?