“Our Gedolim said it’s Ossur to go to the Israeli army. This is from Gedolim from the previous generation. Even if I didn’t personally understand the reason, it’s a Kiddush Hashem to listen to them.”
First of all, the situation today is different then it was in the previous generation. The IDF is willing to have glatt kosher units keeping high standards of halacha. There is no longer an official objective to secularize the members of the IDF.
Second of all, it is not a kiddush hashem to follow a ruling from a previous generation without knowing the reason why. Halachik and Hashkafic rulings of Gedolim (a.k.a. Daas Torah) are not meant to be followed blindly but to be followed based on an understanding of the reasoning behind them (assuming one has the capabilities to follow the reasoning). If you don’t understand the reasons behind the ruling then it is dangerous and foolish to apply it to different circumstances.
I am willing to listen, learn and understand. The original poster asked for an explanation. So far (with the possible exception of Akuperma, pending investigation) the explanations have been lacking. If someone gives me a good convincing pshat behind the purported (it would be nice to see an actual teshuvah) I will accept it.