Even though there is a certain amount of criticism from the religious world on the State of Israel, I think it is mistaken from a few different angles, --mainly by the fact that both Reb Moshe Feinstein and Rav Aaron Kotler held ''the law of the country is the law'' in regards to Israel. [That means it has the legitimate category of a state with all the implication of that in halacha.] So I would say it is great mitzvah to serve in the IDF. If you look into history yoas in their area in the old city.
The book of the Rav of Satmer is meant to show that the State of Israel is not legitimate is based on a midrash about not to go up to Israel "as a wall" against the nations. This ignores the League of Nations and the Balfour Declaration.
