Every slave ship sailing from Africa to the USA sailed under the US flag.
For over 100 years of slavery, it was all done under the US flag.
Abraham Lincoln won the 1860 presidential election, His name was not even on the ballot in 10 states. There were only 33 states at the time so close to 1/3 of the states did not have him on the ballot and he still won. That was the key that started the whole civil war! An election that even today would cause riots, to have a candidate win when he was not even on the ballot in 1/3 of the states!
Yes racial tensions were high and yes the south decided to make slavery a key point of there cause, but when the Civil war broke out it was not all about slavery. Abraham Lincoln himself was "Anti-Slavery" meaning against slavery's expansion, however he was not calling for immediate emancipation.
"I say that we must not interfere with the institution of slavery in the states where it exists, because the constitution forbids it, and the general welfare does not require us to do so." -- Abraham Lincoln September 17, 1859: Speech at Cincinnati, Ohio