Compulsory or voluntary?

B"H

Dvarim (Deut.) Ch. 21:10 reads:
כי תצא למלחמה על איביך ונתנו ה' אלקיך בידך...

Loosely translated:
When you go to war against your enemies, the L-rd G-d will deliver him into your hands...

Rashi's commentary reads:
כי תצא למלחמה - במלחמת הרשות הכתוב מדבר

Loosely translated:
When you go to war - Scripture refers to a voluntary war...

So, the war appears to be one which the Jewish people were not coerced into, but rather one that's fought for other reasons.

However, a homiletic interpretation of "Your enemy" is the יצר הרע (=evil inclination) that a person possesses within. In other words, the scripture says that when you do battle with the evil inclination, Hashem will surly help by helping you defeat it.

Now, battle with one's evil inclination is not voluntary. In other words, one is obligated (and indeed compelled) to fight his/her own evil inclination at all times, as is alluded to in many verses in Torah. (For instance: Dvaim 17:7)

So then, what is this war being fought here? Is it voluntary (as Rashi explains) or is it compulsory?

A possible approach:
Possibly both approaches are true, but they relate to different individuals. The individual who cannot successfully provoke his/her evil inclination and do battle with it and defeat it is told that this is a voluntary war, i.e., "go for it, if you feel up to the challenge." After all, not everyone's up to the challenge and even those that are may end up loosing the battle. So, the battle is tagged as voluntary. Should the evil inclination be kindled and attach the Jew, on the other hand, then of-course the Jew is obligation to defend him- or herself.

An altogether different individual, on the other hand, who is very strong in his yiddishkait and will not be swayed by his/her evil inclination (because his evil inclination cannot exercise control over his/her actions) is commanded to actually provoke and go to war on the evil inclination until it's totally defeated and nullified.

It's also noteworthy that the term used here is על איביך which literally translates to "[go to war] over your enemies", and not עם איביך (...with your enemies) or נגד איביך (...against your enemies.) This would indicate that the Almighty give the Jew all the necessary strength to go to war with his evil inclination, but from the very get-go to be at an advantageous position -- "over you enemies." The evil inclination has no chance if a Jew is approaching the battle-field with the knowledge the the outcome of the battle has already been decided in advance: evil inclination TKO'ed in the first round.

Good shabbos.