"Sorry, in 1858-59 more tariff revenue was generated by Boston ($5,133,414.55) than by the 10 largest southern ports combined ($2,874,167.11). And New York generated almost 7 times as much revenue as Boston ($35,155,452.75)."
The threat to the Union's tax revenue of a free-trade zone between the Confederacy and Europe was not limited to the loss of Southern tariff money, whatever proportion it may have been of total tariff revenue. All of the latter was threatened. European shippers would have significantly shifted the destinations of their exports from Northern to Southern ports. Numerous Northern editorial writers wrote of this threat in an effort to scare up popular support for war from a largely indifferent general population in the North. The port of New Orleans was considered a particular threat because from there the Mississippi would give traders of imported European goods access to an enormous number of potential buyers. Small wonder that the capture of New Orleans was a first priority.