In Sefer Shoftim Perek 3 Pasuk 2 (see Rashi) it says that because the Jews only became worse than the generations that preceded them they had to train in war techniques because G-D wasn’t performing as many miracles for them anymore. According to this, how come Jews nowadays, even the ones that learn shouldn’t be required to train for war and we shouldn't depend on big miracles like the olden days.

We don't depend on miracles and serving in the army in Israel is a Mitzvah (Ramban additions to positive commandments 4) and no Jews are exempt from this important Mitzvah. In some cases if one is studying in Yeshiva he can delay his army service till it suits him according to the arrangements the army has with the Yeshivas in Israel.