By no means the least of the evils of slavery was a dread which had haunted every southern household from the beginning of the government that the slaves might one day rise in revolt and take sudden vengeance upon their masters. This vague terror was greatly increased by the outbreak of the Civil War. It stands to the lasting credit of the negro race that the wrongs of their long bondage provoked them to no such crime, and that the war seems not to have suggested, much less started any such attempt. Indeed, even when urged to violence by white leaders, as the slaves of Maryland had been in 1859 during John Brown's raid at Harper's Ferry, they had refused to respond. Nevertheless it was plain from the first that slavery was to play an important part in the Civil War. Not only were the people of the South battling for the principle of slavery; their slaves were a great source of military strength. They were used by the Confederates in building forts, hauling supplies, and in a hundred ways that added to the effectiveness of their armies in the field. On the other hand the very first result of the war was to give adventurous or discontented slaves a chance to escape into Union camps, where, even against orders to the contrary, they found protection for the sake of the help they could give as cooks, servants, or teamsters, the information they brought about the movements of the enemy, or the great service they were able to render as guides. Practically therefore, at the very start, the war created a bond of mutual sympathy between the southern negro and the Union volunteer; and as fast as Union troops advanced and secession masters fled, a certain number found freedom in Union camps.