What effort was aimed at breaking the will of Southerners?

In 1863, the tide of the Civil War turned with key Union victories at Vicksburg in Mississippi and at Gettysburg in Pennsylvania. In spite of the successes the Union achieved, the South was still deeply entrenched and ready to fight to sue for a settlement or peace and the issue would dominate the 1864 election.

Explanation:

The collapse of white supremacy would be so cataclysmic that no self-respecting Southerner could fail to rally to the secessionist cause, they argued. Secession was necessary to preserve the purity and survival of the white race.

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