Nope, but he made it very clear that he considered the war unjust and illegal on the USA’s part, which certainly wouldn’t help the troops’ morale. He supported his country’s troops fully once it was in the war, but was convinced that we never should have got into it.
As a result he lost the election and didn’t run for office again for a decade, when the issue had cooled down. But Douglass still tried to use it against him in the famous debates.