Answer:
Armies are a symbol of national identity, their stated mission is to defend the nation. If citizens are questioning their national identity, they will find it harder to join the army, to defend a nation they might not believe in, or feel a part of.
This is an issue in countries with different ethnicities. For example: most African and Middle-Eastern countries. A person born in Iraq, but ethnically Kurdish, will not feel represented by the Iraqi army because this army represents the Arab-majority that inhabits the country.