According to Chassidic thought, the war against the seven Cana'anite nations alludes to the ''battle'' of refining one's overtly undesirable character traits [which fall into seven broad categories, stemming from the seven emotional facilities of the Animal Soul - the Nefesh HaBehamis] Consequently, this ''war'' is not relevant to the tribe of Levi, or to those who aspire to their spiritual level - as Rambam writes that this could be ''any type of person - whose spirit inspires him, and he resolves in his mind to set himself apart [from worldly pursuit], to stand before G-d and serve as His minister, to work for Him, and to know G-d'' [end of Laws of the Sabbatical and Jubilee Years].
In contrast, the war against Midian involved fighting against a subtle type of evil which is found in virtually every personality, even those who dedicate themselves as full time ''ministers'' of G-d. Thus, even the Levites and those among the Jewish people who devote themselves ''to stand before G-d and serve as His ministers'' must participate in waging the spiritual war against Midian.
What is the ''subtle evil'' that can plague even the most dedicated servant of G-d? Chassidic thought explains that this is a lack of unity and camaraderie between one man and another, indicating underlying emotions of divisiveness and unjustified hatred. All this arises from a sense of our over-inflated self-importance, which causes a person to be intolerant of others and eventually view them as enemies. Clearly the war against these attributes is very important indeed.
Source: Based on Likutei Sichos vol 28 Lubavitcher Rebbe