Responsa zu Jirmejahu 29:7

There is no question that, in Jewish law, military service, when it is required by the government, must be accepted wholeheartedly by subjects or citizens of Jewish faith. The duty to respect the commands of the government is clearly stated and emphasized in Jewish law. This attitude of respect and loyalty to the government is summarized, for example, in the introductory statement (on page 10) of Isaac Elchanon Spector's "Eyn Yitzchok Hasheni" (who quotes Proverbs 24:21, Aboth 3:2, Jer. 29:7, Yoma 69a - Simon the Just to Alexander). The specific duty to serve in the army is described in detail by the Chofetz Chaim (Israel Meir Hacohen of Radun) in his Introduction to "Machane Yisroel": "It is a great sin," he says, "to evade service in the army."