The Kli Yakar - Parshat Ki Teitzei

In this week's parsha:

•  The Jewish way to fight and win a war
  
•  The REAL sin of Ammon and Moav 

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"If you go out to war against your enemies, and Hashem your God will deliver him into your hands, and you capture his captives" (21:10)

The Kli Yakar infers from the plural form of the word "enemies" in the plural form (as opposed to just one enemy, as implied in the continuation of the verse when it switches to the singular "him") that the verse refers to both your internal and external enemies:

• your yetzer hara
• your actual physical enemies.

 He goes on to explain that once you have battled your yetzer hara, then before you have even readied your heart to go out to battle against a physical enemy, they have already battled your physical enemy in Shamayim.
He goes on to explain why the Torah specifies going out to war:

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When there is no peace between the warriors, then they are already conquered as happened to those wrongdoers of Yisrael in the Second Beit Hamikdash. Therefore, it says "when you go out"....and reduce the internal war within the city, meaning that davka in your going out of the city, you'll spread your hand of war, but within the city, there will be peace among you...and davka upon your enemy you’ll go out and not upon your neighbor whom you love like yourself, and you'll capture the captive of the enemy and will not seek to entrap your friend...because these are all things that cause harm in war.

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The REAL Sin of Ammon and Moav
"Because they did not greet you with bread and water on the way...." (23:5)

The Kli Yakar points out that it doesn’t make sense to keep one's distance from two nations (Ammon and Moav) just because they didn’t go forward to greet you with bread. What's implied here is that Ammon and Moav's primary goal was to cause the Jews to sin.
The spiritual annihilation of the Jews was more important to them than physical annihilation.
They wanted Bnei Yisrael to be hungry so that Bnei Yisrael would be compelled to eat from the occult sacrifices and drink from the impure jugs of wine and habituate the Jews to lewd behavior as summarized in Sanhedrin, that any exhausted and weary person will not discern between the forbidden and the permitted, eating and drinking anything he is given. And that Hashem knew the secret intentions of Ammon and Moav, which are hinted at when it says that "they didn’t go foward to greet you" - meaning that the men didn't go forward to greet Bnei Yisrael; they sent their women instead. The word kadmu is usually translated as "greet," but really means to initiate, to start, to precede, to go forward.

Even though the females played the main role in the illicit behavior, nonetheless the majority of the blame falls upon the men because it is their way to go forward first and the men initiated the spiritual downfall and they forced their daughters to engage in illicit behavior. And that is why Bnei Yisrael allowed the females to live in the war with Midian because they erred in the ruling by distancing themselves only from the males. And this is a worthier explanation than that which all the other commentators spoke of and it resolves all the difficulties that were posed.

Rabbi Shlomo Ephraim of Luntschitz (1550-1619) lived in Bohemia (which is today Poland and Czechoslovakia). He served as rabbi and dayan and wrote several books, the most well-known being his commentary on the Chumash known as the Kli Yakar.
This is my own translation and any errors are also mine.