You should increase it with the help of national focuses directed at the said index. France has a very low national unity index. To level the playing field you should focus on other aspects of the military power and build powerful air forces.
Note the huge slowdown of the Land Doctrine research time. While playing France you must start by changing the rules of recruitment, which will allow you to constantly increase your military capabilities. Hidden behind the Maginot Line they have no idea about the dangers of the German Reich. France lives in a relative sense of security after the end of the World War I.
