What is the definition of a combatant?

Let's say Germany invaded Poland in 1939, annexing the western half of the country and stopped at that. Poles were driven from their homes and fled eastward. As German immigrants flooded into this new part of the expanded German state, those Poles who remained were forced to live under a system which, while not murderous in itself, strictly limited their social political and economic activity, in the interests of protecting "the integrity of German culture." Now let's make things more complex.
20 years later, after the world has become acclimated to the new borders, foreign protests have slowed, and the world has gone back to business, the Polish government, which has retained control in the eastern half of what was their country, joins with their eastern neighbors to oust the invaders in a sudden and bloddy war. The assault fails miserably. Germany by this point has secured the military and financial backing of western Europe, and despite their immense material resourses, the political culture of the eastern states is fragmented and disorganized after centuries of the economic and cultural strip-mining that define all relations between the provinces and any financial and political hub. Germany expands into the new territory, causing a new round of protests and international condemnation which continues to this day, placing limits on its ability to maintain control over the territories, even as small groups of determined and well armed Germans, including families, with tacit and occasionally overt government approval, continue to move in.