On this day

The costliest war in American history began.

It did not have the full support of the people of Union as set out here:

The Copperheads were a faction of Democrats in the North ... who opposed the American Civil War, wanting an immediate peace settlement with the Confederates.
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During the American Civil War (1861-1865), the Copperheads nominally favored the Union but strongly opposed the war, for which they blamed abolitionists, and they demanded immediate peace and resisted draft laws. They wanted Lincoln and the Republicans ousted from power, seeing the president as a tyrant who was destroying American republican values with his despotic and arbitrary actions.

Some Copperheads tried to persuade Union soldiers to desert. They talked of helping Confederate prisoners of war seize their camps and escape. They sometimes met with Confederate agents and took money. The Confederacy encouraged their activities whenever possible.[1] Most Democratic party leaders, however, repelled Confederate advances.

More on the Copperheads here