[Heyworth]

It was also the one most exploitable to keep outside parties from joining in the war in an overt fashion.

Then the states that issued Declarations of Causes for secession probably shouldn't have mentioned slavery over and over again (with maybe a sidelong glance at tariffs, like Georgia, or Mexican bandits, like Texas) as the reason for leaving the Union. It's not like the north had a hard time convincing "outside parties" that the south was fighting to preserve slavery.

[P-40]

It's not like the north had a hard time convincing "outside parties" that the south was fighting to preserve slavery.


Note use of the words "one of" as they are significant.

And yes, slavery as an issue kept much of the support from coutries such as France from appearing.