edit: This bug was mentioned before, and the great developer said it'd be hard to fix it because that requires deep changes in core mechanics.
But because this bug is really frustrating, I am going to keep the text below.
When a nation attacks another, and a third neutral nation happens to be a neighbour of both, the ownership/control of the pixels conquered by the victorious nation is transferred to the neutral nation, effectively separating both warring nations, and rendering small-scale wars impossible.
I also cannot force two countries to ally, make peace or wars quite frequently.