Why did Moshe need to say "If you will not cross to fight..."? Why is it not obvious from the previous Pasuk "Im Ya'avru B'nei Gad ... "?
Kidushin 61a (according to R. Meir): Because a T'nai is valid only if it is doubled 1 (discusses both sides of the condition). 2
Kidushin 61a (according to R. Chanina ben Gamliel): Because he needed to stipulate that if they do not cross, they will inherit like the other tribes in Eretz Yisrael.
See Torah Temimah, note10. Moshav Zekenim: The Gemara also learns from here a. that the positive side of the stipulation must precede the negative one; b. that the T'nai must precede the 'act' (the issue that the T'nai concerns), and c. that the T'nai must be possible to implement. If any of these conditions are not fulfilled, the T'nai is Bateil and the act takes effect. See also Torah Temimah, note 10, who elaborates further.
Moshav Zekenim (citing Sotah 7b): In fact, Yehudah's Niduy 'If I will not bring Binyamin back to you - in Miketz Bereishis, 43:9 took effect, even though he fulfilled the T'nai, because he did not double it.
