[SamAdams76]

The very definition of "civil war" is a war between citizens of the same country.

So yes, it was a civil war.

Remember that the Confederacy was never recognized by the U.S. as a separate country. They couldn't even get recognition from non-belligerents. For example, Jefferson Davis sent envoys over to Europe but they were snubbed because even Britain and France could see that their claims that they were their own nation were preposterous.

[Enterprise]

"Why Lee Invaded Maryland: Lee's greatest objective was that a victory in the North could possibly gain diplomatic recognition from Europe and bring England and France to the aid and assistance of the South. Such a victory might cause the people of the North to question President Abraham Lincoln's leadership and force him to sue for peace.

Why Lee Invaded Maryland

Diplomatic relations are only given to nations. Europe viewed this as a civil war, not a war of the Southern Nation against the Northern Nation.