The bombardment of what fort in April 1861 ignited the Civil War?

Fort Sumter in Charleston Harbour. The Union garrison evacuated the fort, and Lincoln called for new volunteer troops.

This was the equivalent of declaring war. (There was, of course, no official declaration, since Congress did not recognise the Confederacy as a sovereign nation.)

Fort Sumter, being the property of the US, was the first step to the US Civil War. Mr. Begg is correct that it was aftert Fort Sumter that the US Civil War began.