ORIGINAL: mikmyk
There's a bunch of existing scenarios with fictional opfors. Go right ahead.
One trick for a plausible quasi-fictional opponent...
Start with an actual historical event like a coup or a revolution or widespread protests. Assume something goes very wrong and the country descends into full blown civil war. At this point, one or more regional or global powers decide to get involved, either to keep the civil war from spilling over into their own country, or simply to take advantage of the situation. However, if one country takes the side of the rebels, another powerful country might take the side of the existing government. And things escalate from there.
For example, let us say that 2014 rolls around and the Syrian civil war is getting worse. Turkey decides to step in and help some of the rebels. It starts making air strikes against targets in Syria. At this point, Russia might threaten Turkey. Turkey, a member of NATO, yells for help. Suddenly, we have the beginnings of a global war...
Another example...a bit more "out there"...in 1976, there was a coup in China against the Gang of Four. What if it had resulted in another civil war in China. What if one faction decides the easiest way to win is to cut a deal with the United States and Taiwan? By 1979, the Chinese War is full swing, with American aircraft flying alongside Golden Chinese forces and their Taiwanese allies in their struggle against the Red Chinese!