The immigration ports of entry were only open in the North because the Southern ones, such as New Orleans were in Union hand in the early part of the disagreement. The Southern ones I've encountered were NOLA, Mobile, Charleston and that hub area with Virginia and Maryland. Boston and NY were pretty much the active entry points during the war. Maybe the Union wasn't just thinking of a supply line for goods for the South when they took the Southern ports but also were thinking of the body in uniform supply count. No reenforcements a comin over the sea for the Gray. Had there been, it may have made a difference. We'll never know. I'm not including San Francisco because of the time element and I'm not sure how many different nationalities came in through that entry point.
Pam