There is an expression used in yesterday's Parsha, in connection with the trumpets, the חצוצרות. When the Torah alludes to a future war, it is referred to an enemy called הצר הצורר אתכם. This is translated as "an enemy that is oppressing you. This is the only time the Torah uses such an expression in describing our enemies.
The Rambam in Hilchot Melachim defines what is a מלחמת מצוה, or an obligatory war. He says that this applies to our war with Amalek, the war with the Seven Nations, and הצר הצורר אתכם. Here we are given a clearer description of this term.
Any nation that lives among you and wishes to drive you out of the land, falls under the category of הצר הצורר אתכם. Rav Yisrael Meir Lau, the chief rabbi of Tel Aviv, once said that there is no doubt that our Arab enemies are today's הצר הצורר אתכם. All of the rules of מלחמת מצוה apply in our war with them, except one.
Unlike Amalek and the Seven Nations, where the Torah commands to kill them all including women and children, this does not apply to הצר הצורר אתכם. We are commanded to defeat them in order to remove any threat to our security in our land.