Grant must have been drinking when he issued that order. It affected ALL Jews "as a class" including Jews serving in the US Army.
The author of this book seems like someone worth listening to on this subject:
"Dr. Jonathan Sarna is the Joseph H. & Belle R. Braun Professor of American Jewish History at Brandeis University and Chief Historian of the new National Museum of American Jewish History.
Dubbed by the Forward newspaper in 2004 as one of America's fifty most influential American Jews, he was Chief Historian for the 350th commemoration of the American Jewish community, and is recognized as a leading commentator on American Jewish history, religion and life. In 2009, he was elected to the American Academy of Arts and Sciences"
I agree it would have been impossible for me to support the South in that struggle.
Apparently, there was a movement in the North prior to Fort Sumter that called for letting the Confederacy rot and invading Canada instead.