Obligatory war in Jewish law

Milḥemet Mitzvah or in Biblical Hebrew Milḥemeth Miṣwah (Hebrew: מלחמת מצווה, "War by commandment") is the term for a war during the times of the Tanakh when a king (of the Kingdom of Israel) would go to war in order to fulfill something based on, and required by, the Torah without needing approval from a Sanhedrin, such as war against Amalek. milhemet mitzvah tended to be invoked in defensive wars, when vital interests were at risk...Questions about the halakhic justification for war again arise in modern times in connection to military operations of, and service in, the Israel Defense Forces. To the extent that such operations are perceived to be self-defense, they are viewed as examples of milchemet mitzvah
Maimonides Mishneh Torah Melachim uMilchamot - Chapter 5.1

Amalek, as described in the Bible was a barbaric, terrorist nation that attacked civilians, women and children from the rear, as opposed to sending their soldiers to attack soldiers. This is a parallel to the way Hamas and other Islamic terror organizations