When did Lincoln declare war against slavery?
There was no formal declaration of was against the Confederacy,
which can ONLY come from congress... as the Union deemed the
secession illegal, it never occurred; to do otherwise would have
legitimized the Confederate States of America opening the door to a
possible recognition and assistance by Great Britain.
Secondly, as ONLY congress has the power to declare war, the
Civil War was, in effect fought using what is known today as
"Executive Powers".