Every one of our presidents have used force "to compel obedience to federal law." Our Constitution obligates the president to "take Care that the Laws be faithfully executed . . ."
(I'll bet that they tried to teach you that in high school civics.)
My point is that he was using force to compel the states to remain in the Union, not to end slavery. He wasn’t sending his army out with the mission of freeing the slaves, which is most likely what my high school civics class was teaching.
While Lincoln hated slavery he also respected the rule of law, at least occasionally, and he repeatedly said that he didn’t have the authority to order slaves to be set free. He eventually did it as a war measure affecting only the States which were in rebellion, which of course the British had also done during the American Revolution. But slaves in the states that remained loyal to the Union remained slaves.
Frederick Douglass did a good job of describing Lincoln’s opinion of blacks and slavery in a speech he gave on the 10th anniversary of Lincoln’s death. Douglass knew Lincoln personally. Douglass said that he and his fellow blacks were afterthoughts in Lincoln’s decisions. And while he was eternally grateful for what Lincoln did he harbored no illusions that the war or any other great decisions made by Lincoln were done with blacks first and foremost in his mind.