"One of the boldest exploits of John Brown was his escorting, in 1858, a band of twelve slaves from Missouri by a devious route of well nigh 1,000 miles to Windsor, in Canada, in mid-winter, in spite of a reward of $3,000 for his arrest. This raid excited great alarm in Missouri. Many slaves, as a consequence, were sold south and others escaped. John Brown's policy, he himself avowed, was to destroy the money value of slave property by rendering it insecure." [Source]
The story of a John Brown Canadian associate here.