The Southern armies might have won a guerilla war, but marching en masse to battlefields against an opponent with three times your population is a sure way to lose.
Only if you believe that Lincoln was acting in the wrong. If you believe that the southern rebellion was illegal, that the southern acts of unilateral secession were unconstitutional, and that Lincoln was in the right in opposing the southern rebellion then that doesn't make the Northern cause right just because they won. And it wouldn't make the Northern cause any less right if they had lost.
The Southern armies might have won a guerilla war, but marching en masse to battlefields against an opponent with three times your population is a sure way to lose.
But the southern leadership started the war firm in their belief that one good old southern boy could whip ten of those miserable, immigrant Yankees. They badly underestimated the resolve of the Union and overestimated their support for their cause.