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Several Northern states passed laws that forbade local authorities from helping apprehend fugitive slaves. In addition, the Northern public often actively helped fugitive slaves. The net result was that Southern slaveholders frequently complained about the lack of enforcement of the Fugitive Slave Act. Absent the restraints of Northern public opinion, slaveholders expected that the Confederacy as a whole would be more diligent and effective in controlling their slave population and preventing them from fleeing captivity.

Most if not the vast majority of them knew full well it would be impossible to secure their borders or to prevent slaves from crossing over those borders. There were only 5.5 million White Southerners. The borders of the original 7 seceding states ran for 1500 miles and obviously mid 19th century technology was nothing like modern technology. Secession meant the rapid end of slavery and most knew it.

And again, not only the majority but the vast majority of White Southerners did not own any slaves. Protection of slavery was obviously not what was motivating them.