In regards to a simple theme, there is no doubt that the major theme in Suite Francaiseis is war. Set in Paris (and other towns in France) in the very early 1940s, there is no way to escape this theme, for at that time the Germans were busy taking control of the country. For the first proof of the theme, you have to look no further than the third sentence:
It was night, they were at war, and there was an air raid.
The first part follows five families fleeing from the German bombs dropped on French cities. Disaster ensues. There are many passages that reveal the horrors the fleeing pedestrians encounter.
Still at some distance, great guns were firing; they drew nearer, and every window shuttered in reply. In hot rooms with blacked-out windows, children were born, and their cries made the women forget the sounds of sirens and war.
The second part happens some time later (a year, to be precise) in order to deal with how two different kinds of families deal with the German occupation.
Luckily, there isn't much in the way of description of military violence. Although there is suffering, death, and loss, the actual violence is not what this novel is about. Instead, Irene Nemirovsky focuses her attention on how the war causes great tension and discomfort for local French citizens. In truth, it is about the French citizens' struggle of survival during the occupation of the German forces.
I would add, however, that there is also the additional theme of compassion within wartime found within Suite Francaise. The citizens often help each other in many ways (both physical and mental). They occupy themselves pretending to obey and respect the Germans while plotting to thwart their efforts in any way possible.
Therefore, even when minor themes appear, war is always paramount in Suite Francaise.