Regarding slavery, i think it would become obsolete with technological development, specially in the crop fields and it would dissapear with the passing of time.
About the international support and the dependancy of the CSA army on it, i meant that, compared with the north, their army was much more international.
Now, on with the history:
World War I (only major participants listed):
Entente:
-United Kingdom
-France
-CSA
-Italy (Latecomer because at first they were aligned with the central powers)
-Russia (Until Russian Revolution sparks)
Central Powers:
-Germany
-Austro Hungarian Empire
-USA
-Ottoman Empire
Important Neutral States:
-Texas
-Switzerland
-Mexico
-Scandinavian Countries
On August 1914 the First World War started. In American territory, the CSA controlled the states of Virginia (where their capital, Richmond, was located), North & South Carolina, Georgia, Florida, Alabam, Tennesse, Mississippi, and Arkansas. Those where the territories that the pact of Nassau (the one which ended the Civil war in 1865) granted fully control to the CSA, while the USA got the rest (the only difference with OTL is that West Virginia would be named Allgheny).
However, the CSA still mantained claims on Missouri, Kentucky, Allgheny and Maryland, apart from Kansas. And, while they got support from the British Empire, in a secret telegraph, (kinda like the Zimmermann telegraph in OTL) in 1915 the CSA waged a war with the USA hoping to retake the aforementioned territories. At first they succeded, partially, because they could retake the southern half of Missouri, and greatest parts of Kentucky, while keeping a deffensive stance on the east.
Meanwhile, in the north, the US Army took the initiative and invaded Canada, but they understimated the Canadian winter and, on winter 1915 they suffered a great defeat against Canadian troops in Calgary, on the west and in Ottawa in the East. This gave the chance to the Canadian army to counter attack and, despite initial success on US lands, they were driven back and the frontlines on the north almost got inmobile because both sides took a deffensive stance, the Canadians because their resources were rather scarce to perform a massive offensive, and the US because they centered their efforts on holding up the Confederates at the south.
The southern fronts almost didn't change with some occasional skirmishes here and there and the typical failed offensives that were very typical of the First world war.
Some examples of the weaponry used by the CSA in this war:
Sopwith Camel:
Sopwith Triplane:
And, on land, the Mark V tank:
The war ended like in OTL, in November 1918 and with heavy human casualties for both sides and handing over the control of southern Missouri to the CSA.
However, as i said before, the technological advance made the slavery really expensive, and this gave freedomship to hundred of thousand African Americans in the CSA, however, as nobody wanted to employ them (they were, in the 20s highly discriminated), a very serious crisis emerged and, that crisis, mixed up with the crack of 1929, favoured the ascend to power of the fascism in this country...
And i stop here because i have plans for the World War 2.
EDIT: I apologize if i sound racist. It wasn't my intention. I only tried to tell an alternate history.