Why was it so difficult for the U. S. military to combat the tunnels of Cu Chi?

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Explanation:

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bloodiest single day of war   |   gettysburg

where the civil war ends for good   | appomattox court house

the first major land battle; south wins | bull run/manassas (va)

gives union control of the mississippi river |   vicksburg

lee’s first tactical defeat; big win for union |   fort sumter1

with lincoln's re-election |   atlanta

south wins

splits confederacy in two for the duration of the war,

antietam