Slavery was lawful in many states when Lincoln invaded the south. In his inaugural address he made it abundantly clear that he had "...no purpose, directly or indirectly, to interfere with the institution of slavery in the States where it exists. I believe I have no lawful right to do so, and I have no inclination to do so.".
In his own words he admitted that his purpose was to preserve the Union. Any effort to abolish slavery would require the ratification of an amendment end slavery in the states where it existed.
The secession of the southern states would have caused irreparable harm to the northern economy. In order to save the Union it was necessary for Lincoln to prevent the secession of the southern states.
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